Hi, I've noticed an increase in posts concerning topics already tackled multiple times in the past.
Trying to compile everything in one thread so it's easily accessible.
Note: I've tried to leave aside all subjective questions (e.g. "Is it a good game?").
Also, make sure you browse through other users' contribution further down!

Before we start one thing you should keep in mind, above anything else...
Guild Wars isn't WoW without monthly fees
Please make sure you repeat that for yourself a few times, and that you understand the implications. Otherwise you might end up very confused and/or upset.

Contents:
What kind of game is Guild Wars?
Do I need all the expansions to play Guild Wars?
Which campaign is best to start with?
Can I travel to campaign Y/play with friends who have campaign Y with a character from campaign X?
Where do I find information about (insert name of item/boss/whatever here)?
Will GW run on my comp?
Is there a trial version?
Is PvP balanced? Is the game casual friendly? Is there high-end content?
Any other things that might appeal to casual players?
What does the game offer for more hardcore players?
What's the best profession/profession combination/skill template for X?
Where's the auction house? Hey I can't jump! Can I buy a mount?


What kind of game is Guild Wars?
That is somehow hinted by the name; in a nutshell, the primary focus is skirmish PvP between groups of players.
Now, the PvE portion is indeed extensive, to the point that some players never PvP at all and are fine with that, however whevener you feel that something "isn't like what I saw in *insert game here*", keep the above statement in mind.

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Do I need all the expansions to play Guild Wars?
First of all, Guild Wars consists of three campaigns and one expansion. Campaigns (Guild Wars - also known as Prophecies - Guild Wars: Factions, and Guild Wars: Nightfall) are stand alone games. You can start with any of those, get the others later on, and bind them to your original account (This is done by following the instructions on the log in screen: "Create Account or Add Access Key"). The advantages of having all campaigns on one account are: 2 extra character slots per campaign, increased in-game storage space, more ground to cover, more skills to choose from (and I stress the notion of choice, more on that later), access to new professions (in the case of Factions and Nightfall) as well as campaign specific features.
Say you have Prophecies and then add Nightfall to your account. The characters you made in e.g. Prophecies get a quest to travel to e.g. Nightfall, where they pick up the storyline a few missions after "natives" start (getting to level 20 in Factions and Nightfall goes faster than in Prophecies). If you create a Nightfall specific char, it will be the other way around, they'll start with specific quests, and later get a quest to travel to Prophecies, where they pick up the storyline in Lion's Arch.
Storage is shared across your whole account. Since a few updates ago, the more campaigns you own on one account, the bigger storage you get.
All three campaigns also share the following:

• PvP arenas (not the Factions specific ones, I mean Random Arenas, Team Arenas, GvG and such)
• Guild Halls
• Fissure of Woe and the Underworld
• Some skills (referred to as Core skills)
• The 6 original professions (Core professions)

• "Standard" fantasy setting (West European)
• 6 professions, referred to as "Core" professions and available in all 3 chapters. These are: Elementalist, Mesmer, Monk, Necromancer, Ranger and Warrior
• Long PvE campaign (in relation to other chapters)
• Missions have a main objective and a bonus (unrelated) objective
• Slowest levelling pace, you spend roughly 1/4th of the PvE storyline at max level
• Skills can be obtained from quests throughout most of the PvE campaign
• Possibility to rush through PvE maps freely
• Access to henchmen only (NPC teammates, which you can add to your party in case you can't or don't want to have live players in your group)

- Factions:

• Based on Asian lore
• Adds 2 exclusive professions: Assassin and Ritualist
• Shortest PvE campaign (again in relation to other chapters), focuses on PvP between 2 factions (hence the title)
• Fastest levelling pace, you can reach level cap before accessing the 3rd mission (out of 13)
• No bonus on missions, though better reward the faster you complete them; also introduces Challenge missions, where you have to obtain the highest score you can completing a given objective within a fixed time-frame (those are unrelated to the PvE storyline)
• A handful of skills are given from quests on the starter area, rest must be bought; quests and missions give higher rewards than in Prophecies to compensate*
• New explorable areas are accessible only once you complete given missions
• Access to henchmen only

- Nightfall:

• African theme
• Adds 2 exclusive professions: Dervish and Paragon
• Long PvE campaign, on par with if not longer than Prophecies'
• Levelling pace somewhere in between Prophecies and Factions
• Missions with bonuses; however, unlike Prophecies, you can't do the mission first and come back for the bonus later. It's more about completing the mission with extra challenge.
• A handful of skills are given from quests on the starter area, rest must be bought; quests and missions give higher rewards than in Prophecies to compensate*. Note that even though Assassin and Ritualist are Factions-only professions, Nightfall offers new skills for those 2 classes
• New explorable areas are accessible only once you complete given missions
• Another notable feature is the addition of Heroes, which are henchmen you can customize (skills, equipment) and level up

* A few skills for core professions are obtainable through quests in Prophecies for Factions and Nightfall characters once you unlock travel to the first campaign
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Can I travel to campaign Y/play with friends who have campaign Y with a character from campaign X?
Yes, though there are some conditions to meet.
For the record, there are no hard server boundaries in Guild Wars. Players are automatically sent to districts which correspond to their area (e.g. Europe, America, etc.) and in the case of Europe, this goes further to language districts (European English, European German and so on). Your home district can be changed in the account options and is thus based on the information you provide. There is nothing that prevents you from going to other districts (besides ping). Also, there is an International District where you can meet any player from any part of the world.
Any character from any campaign can travel to any other chapter provided you 1. own that chapter 2. have bound it to your account.
When trying to play some PvE with a friend, you two need to own at least one campaign in common. In PvP it doesn't matter, all arenas (aside from Factions-only skirmishes) take place on the same map, shared by all campaigns.
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Where do I find information about (insert name of item/boss/whatever here)?
Your best bet is the Wikis:
Fan made: here
Official: here
Note that the official wiki is now accessible once in game. Type /help in any chat channel.
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Will GW run on my comp?
GW has never been really hungry in terms of computer horsepower.
Taken from the official FAQ about Prophecies:
Minimum System Specs:

• Windows XP/2000/ME/98
• Intel Pentium III 800 Mhz or equivalent
• 256 MB RAM
• CD-ROM Drive
• 2 GB Available HDD Space (Note: you will need more if you have several campaigns)
• ATI Radeon 8500 or GeForce 3 or 4 MX Series Video Card with 32MB of VRAM
• 16-bit Sound Card
• Internet connection
• Keyboard and mouse

Recommended System Specs:

• Windows XP/2000/ME/98
• Intel Pentium III 1GHz or equivalent
• 512 MB RAM
• CD-ROM Drive
• 2 GB Available HDD Space
• ATI Radeon 9000 or GeForce 4 Ti Series Video Card with 64MB of VRAM
• 16-bit Sound Card
• Internet connection
• Keyboard and mouse

Of course, the more the better. Here is a link to a benchmark done a while ago by GameSpot.
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Is there a trial version?
A trial DVD is available. Apparently, trial keys shipped with Game of the Year editions too.
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Is PvP balanced? Is the game casual friendly? Is there high-end content?
I thought these questions overlapped each other in some respect, so I decided to put them together and add some specific points below (even if the first one might be close to subjective and depends on the skill updates/new additions).
PvP is pretty much the core of the game (even if it still offers a healthy amount of PvE). The developers focus on PvP aspects when implementing or rebalancing (yes I know, some would grind their teeth and yell "It's called Nerf!" ;) ) skills/professions. Hence you shouldn't expect any "I win" button in game.
Also, among a pool of hundreds of skills (across all classes, the game currently boasts over 1000 skills), you can only have 8 equipped at any time. Each of these skills also has at least one more or less obvious counter. Think of it as an elaborate Paper-Rock-Scissors, or as others mentioned on this forum, a deck of Magic: The Gathering. Basically, you need to find synergies between your own skills and your team members' own setup. Still this is only the tip of the iceberg when it comes to PvP strategy.
Another thing is: Guild Wars isn't gear-centric. The most expensive armour, which requires to complete a lengthy chain of quests in a tough area, gather a fair bunch of rare materials, and comes with a hefty price tag, doesn't offer more protection than the basic max level gear, available from crafters in major towns or from collectors throughout the game.
Same goes for weapons, the rarest type won't deal more damage than more widely available items with perfect statistics and modifiers. Most "Green" weapons (which are fairly rare drops from specific bosses and have pre-determined statistics) have counterparts, i.e. you can obtain the almost exact same weapon by combining salvaged parts together.
Even modifiers for weapons and armour are either conditional or come with a tradeoff.
So why bother with rare stuff? Status. What differenciates higher grade items from common ones is their appearance and scarcity.
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Any other things that might appeal to casual players?
Listing stuff that might already have been written before, though for the sake of quick access:

• Content outside towns is instanced, meaning you don't have to wait for a specific mob to respawn in order to finish a quest for example
• Map travel: once you've been to a town/outpost you can go back there in a matter of seconds using your map
• Henchmen and Heroes: can't find people to do this or that quest? No time to wait to find someone? Recruit NPCs to flesh out your group instead
• Random arenas and in the case of Factions, Fort Aspenwood and Jade Quarry: quick PvP skirmishes in random teams
• Again, low level cap and no equipment that makes or breaks your characters
• Possibility to create PvP-only characters with max level and armour, skills and upgrades can be unlocked by trading Balthazar Faction points, which you earn during any PvP match
• And of course, no monthly fee: no stress about not getting your subscription worth playing sparsely

• A good start is unlocking all Elite skills
• Ascended armour (costs ten times more than common armour, piece, material excluded)
• Titles: oh brother, this is almost endless... Those are tags you can display under your character's name to advertise your achievements. Here are a few examples. PvE titles include Explorer for uncovering maps, Protector for beating each campaign with the highest mission rewards, Wisdom Seeker for identifying golden items, Treasure Hunter for opening high-end chests; PvP titles like Gladiator for consecutive streaks of flawless victories in random or team arenas; slightly more RPG (if I might say so) titles like Drunkard or Sweet Tooth, for spending time drunk (no sh!t) or eating candies, respectively...
• Hard mode: recently introduced, it allows you to replay the game against beefed up opponents. This also unlocks new titles (Guardian for missions, Vanquisher for explorable areas)
• Elite Missions: Sorrows Furnace and Tomb of Primeval Kings in Prophecies, The Deep and Urgoz's Warren in Factions, Domain of Anguish in Nightfall. Count on a few hours required to complete those
• Fissure of Woe and Underworld: high-end zones, long quests, shared by all 3 campaign (i.e. you only need to own one chapter to have access to those)

Another Frequently Asked Question:

1) How is the Guild Wars community?

 (note: this information is obviously not based upon verifiable/measurable items, it is one person's view who has played since GW: Prophecies came out)

The GW community is pretty much a cross-section of real life with the sometime unfortunate inclusion of the anonimity of the internet.  Just like it real life you will find great people, friendly aquaintences, jerks, scammers, random do-gooders, young 1337s, old dogs, boys/men and girls/women.  Everyone in a region are interconnected within a single server and you can even connect with people in other regions through the use of the international districts.  You will run across hundreds and thousands of people in your time each day if you even hope through only a couple districts in hub areas.  The key to the game is surrounding yourself with people you enjoy being around, and this is done by finding a quality guild.

Just by the name of the game you can see the developers were/are looking for people to find a guild and a comfort zone for their own little community.  Finding a guild you enjoy can have a direct correlation on the amount of enjoyment you draw from this game.  It can be a long, dark and lonely road for those trying to go through it by themselves (although it is entirely possible to do so for those that choose to).  On the flip side joining a guild you enjoy gives you the chance to interact and make friends with people on a consistent basis.   Because many people join guilds and interact with only those people, an alliance aspect was created with the release of GW: Factions.

An alliance is a group of upto 10 guilds.  With the alliance chat feature, you can communicate with anyone in you alliance and visit any of their guild halls.  This has the potential to allow you to play with 1000 people (10 guilds of 100 members) consistently.  This means more opportunities to find a person doing a similar quest or just looking to help whenever you ask.  Within GW: Factions you join alliances loyal to one of the two warring factions in-game (Kurzicks or Luxxons) and you battle for control of towns which the top alliances can actually own.

There is continous action across all three games and updates are given across all three.  You can travel back and forth between all the regions, so that keeps players going back to Prophecies as much as Factions.  Of course players do move on, but you can start a new character in GW: Prophecies and still run across many many others.  With the recent addition of Hard Mode, that is bringing a lot of people that went to mainly GW: Nightfall back to complete missions and get different rewards in the previous 2 chapters.

Most people have to and should decide for themselves about the GW community.  It is often accused of being immature and adolesent but that is just the way most online games are nowadays.  You have your mixture of good people and people you would never interact with in real life.  Again it is all based on who you choose to surround yourself with.  With the ability to put people on your friends list or ignore list, you can choose to see the in-game community however you like.

Have fun see you in game.

Nice topic, I approve whole heartedly.

Move to sticky seconded.