I'm going to write down some notes here from my observations from playing both gameplay styles fairly extensively in mmorpg space with friends.
Sandbox mmorpgs are so broad in their scope that self directed player activities flourish to new heights. If the game allows player invented professions and pure sandbox play- then we lose the "sports gaming" feel.
See, the advantages/disadvantages of World of Warcraft over Sandbox is very subtile. You might not notice unless you play both types of games with friends. In WoW, you are traveling through a narrow funnel. Your activites are focused towards earning XP first and foremost. But because your activities are sooo directed, you and your friends will do the same thing- grind Quests together, etc.
In a sandbox title, your activites might become a lot more broad. We lose the "sports gaming" funnel and become much more broad. Players tend to seek out their goals. The pure crafter types will simply craft, basically ignoring both PVE/PVP. The pvpers will leave the safe areas and venture out into risky terriority to make a name for themselves. The Achiever types are more likely to hit up PVE and perhaps also seek out PVP if thats where they perceive the greatest potential for wealth.
As you see, the main difference between the two styles of gameplay is that players are much more self directed. Give players the ability to become anything they want to really be then they will pursue their goals, perhaps only grouping with friends here and there for comaradie. In games like Everquest, we grouped with friends for survival and earning XP.
Patches / Expansions-
The patches we receive in EVE Online aren't content driven per se. Sure, they add some new Agent missions here and there, ships, and other elements but for the most part the patches add "toys". Also as we have noticed, they had time to pursue a graphical revision which most content driven MMOs dont have the man power to do (or perhaps it's just way too much content to revise in traditional mmorpgs).
Patches for Content Driven MMOs normally include new land masses, more gear (aka vertical progression), and just a plethora of content. The developers are sort of running up a mousewheel, no breaks. My artist friend that works at Blizzard was getting worked to the bone it seemed. Barely had time to hang out and kick it (and this was after TBC expansion).
Socializers thrive a bit better in sandbox games due to the loose barriers put inplace (lack of hard defined Levels). Grouping between veteran and newbie is actually possible in Sandbox games. I think sandbox games have the biggest potential to enrich social interaction bar none. Look at Second Life, its a role players / socializers dream.
Player Versus Player -
Sandbox games normally encourage freedom of choice and loosely defined boundaries. They also encourage living a virtual life. Since the restrictions and boundaries are loosely defined in sandbox titles, the pvp aspect emerges in almost every aspect of gameplay. As sandbox titles evolve, we see even more creative ways to remove hard defined boundaries and put in place more natural restrictions (like NPC guards that can be killed, etc).
Linear MMOs are moving in a different direction. They have discarded the virtual world feel and instead embrace Instancing, Arenas, Capture The Flag, etc. They are basically building rigid boundaries between PVE and PVP.
Player owned Lands and Housing -
Much more likely to emerge in a sandbox as we have already seen in the past. Sandbox games usually strive to allow players to impact the world around them versus Linear MMOs which sort of strive to provide the Illusion of Impact via Instances. Sandbox games usually avoid Instances and instead strive for fully populated worlds run by players.
EVE Online has went to the extent to provide only one server that all play on. Consequences can be dire and player Impact is common place.
Items have much more importance in Linear games. Your gear is a symbol of who you are, your player skill, and your accomplishments. In sandbox games, your items that you use are merely tools. In Classic Ultima Online, there wasnt a "number system" attached to items per se (pre-AoS). In EVE Online, your ship is merely a tool that any player can build for you. Most corps go so far as to provide Ship Replacement programs. In linear MMOs, you usually raid for the best gear. In Sandbox games, most items are player crafted
Items are usally all somewhat viable in a sandbox rpg. Vets will still pilot a frigate (newbie ship) in EVE Online because its faster then a vet ship and cheaper. Thus, newbie crafters will find a market for their items because all items usually have some value
In a linear MMO, if all of our players are mid-high levels then obviously newbies usually find no market to sell their wares. Linear MMOs must cater to newbies to survive literally this way all levels will enjoy rich teaming and auctions.
It's not hard to figure out we arrive at a self-balancing system in sandbox mmorpgs. Players will be loathe to wear their overpowered gear for every battle due to item decay / DP. In a linear MMO, I will always, always wear my best items. Since gear is another form of progression, my items in a linear MMO will give me X number of levels advantage making PVP a slaughter usually.
Linear MMOs will pretty much always have poor economies that suffer Mudflation. Items are a symbol of who you are thus Developers are loathe to take that away. Items mean everything. Thus, linear MMOs are more subject to adding on more and more gear with each expansion. It's a never ending race for the best loot. World of Warcraft PVP is at Season 3 now. Each season adds new gear my friends tell me. For PVE, they are always adding new top end dungeons
Economy is a lot more important in a Sandbox mmorpg. EVE/CCP for instance has economists on staff. Player crafted goods is pretty much always a must for this system. Item decay also usually accompanies it. Players truly immersed in the universe realize that these items are merely just tools. However, players from Linear MMOs view items with much more importance and worship.
This is why Sandbox players and Linear players clash so much. The mindsets are totally different and we really dont have a whole lot of insight or information to share. Our gaming styles are completely different. What would work for a linear MMO would be a disaster in a sandbox usually. Vice versa.
Death Penalty -
Both types might include a death penalty to get a risk/reward system happening. In a sandbox MMORPG though, they are more likely to explain death and make it logical. In eve online, death is explained via clones. A harsh DP is more likely to emerge in a sandbox mmorpg in which is trying to pursue a more realistic economy. In a linear MMO- emphasis on economy is downplayed. The best gear is earned from dungeons and raids. Therefore, the need for a harsh DP is less sustained.
Player vs Environment -
This is much more emphasized in a linear MMO because this model strongly relies on the "carrot chase". Loot is just really another form of vertical progression. These types of games are also much more likely to pursue "Instant Gratification" and other material items.
PvE is much more emphasized here because Developers view "Levels" as somewhat counterproductive to PVP. The presence of Levels is what will cause the harsh division between PVE/PVP as well since newbie players are helpless
In a sandbox title, levels probably wont exist (Starport) or they pursue the motto "All items are created equal". So, a newbie in a sandbox game can partner up with other newbies and kill a vet like we see in EVE Online / Starport. In a linear MMO this just cant happen
There is no way for PVP to provide a rich experience in such a model without the heavy use of Instancing, Arenas, and other venues. PVP will more then likely be partitioned by Levels / Rank to keep the competition fair. Fairness, ironically, is what the goal is for linear MMOs.
Grouping can actually flourish much better in sandbox games. Due to the natural restrictions, there is not a hard rigid defined group limit. So the more the merrier. We have lost our "sports gaming" feel we have in Linear MMOs in which we can only bring a set number of players.
However, ironically, due to the heavy freeform nature grouping might actually take a hit. We lack the "funnel" traditional linear MMOs have. Activites are too broad. Crafters dont have to kill anything in a sandbox mmorpg. They can simply get a loan from a friend and start investing this capital into their venture. In a linear mmorpg, we are much more likely to see more grouping because the activites are so restricted and funneled.
This can be fixed for sandbox mmorpgs to an extent.
Sandbox mmorpgs emulate virtual lives. Thus, the Rules of Engagement are in play. In EVE Online, I've never been on an op where they told us where we are going. You dont find out til you arrive at the destination.
In a linear mmorpg things are a lot less serious. Guilds are less suspicious of new players. They usually dont think the new player is a potential spy. In EVE Online, this is a constant worry making it tough for some players to find a home
Alternate Identities -
Sandbox games will normally not worry so much about giving players many slots to make an alt. Linear MMOs will always encourage the generation of "alts" (making multiple characters). This is their model. All levels must always have activity to present a healthy population. They can do fine if all pop is high level but rerolling would be lonely you see due to barriers between veteran and newbie
Thus, linear MMOs will always encourage alts. City of Heroes gave rewards once you reached highest level and allowed you an exclusive new race. World of Warcraft I hear will do the same shortly. This is their model....
Sandbox mmorpgs, due to barriers between vet and newbie being loosely defined, suffer much less from this problem. Thus, they normally dont worry so much about alts. EVE Online provides only 3 slots and most players only use 2 at best. City of Heroes has bout 9 slots and World of Warcraft offers a lot of slots as well.