The Abyss - Levels 25+
Before you hit level 25 and gain access to the Abyss, there are a few different ways you can experience PvP combat. However, that'll largely be in the Arena in your capital city - which, in my experience, rarely has people in it - or in duels. Around level 20, you'll get yourself into an area that can be invaded by the other side, and you can invade their lands by traveling through Rifts that open randomly in the world. However, even then, you still won't see much in the way of PvP, but you can find it if you look for it.
This drastically changes once you hit level 25.
When I entered into the Abyss, it felt like I stepped into a different game. The classes were the same, the combat was the same, but the focus of it was entirely different. I could happily quest and attack NPCs in the Abyss - and get an awful lot more experience there doing so than I could in the rest of the world - but now there was one huge difference: there were members of the other faction around the world that could drop down upon me in an instant.
The Abyss is an area between the two halves of the world that is shared by both sides. Both Asmodians and Elyos hunt and quest there, and most importantly, they battle over great fortresses. While the main focus of the Abyss is on faction-based PvP, and there is plenty of that going on at all times, the real highlight is besieging a Fortress.
A fortress cannot come under siege at any random time. They only become vulnerable at certain periods of time - which might be a real pain if you can't play during the times they are vulnerable. However, it also means that when a Fortress is open to attack, everyone is there. We are talking battles of purely epic proportions, with hundreds of players on both sides partaking in the fight.
The actual mechanics of the sieges are similar to Dark Age of Camelot, or Warhammer Online. You are presented with a castle and some doors to batter down with siege equipment. You then head inside to start battling with whoever is defending it - which can also be members of an NPC faction, the Balaur - and you work your way down to the raid-boss fortress guardian, below the surface of the castle. All the while, you'll need to fend off members of the opposing faction who are trying to stop you at any cost.
Notably, there are a few interesting tweaks to this system. For one, you can purchase mobile bind stones for yourself, your group, or alliance - a gameplay element which gave me flashbacks to PlanetSide. These can be placed down by either attackers of defenders, but can also be destroyed by the enemy, creating another layer of strategy. Also, you cannot fly into the fortresses without destroying what is, effectively, a force field generator inside the castle. Finally, there are artifacts near fortresses that, if captured, can provide powerful bonuses to whatever side did the capturing - including triggering large AoE spells which can exterminate enemy forces.
PvP seems to be rather well balanced. All of the servers have fairly even populations between the two sides, thanks to NCSoft prioritizing the issue. (This created a major headache for people attempting to get onto certain servers at launch.) Some classes are noticeably weak in 1-vs-1 PvP, but there isn't much of that in the Abyss anyway, and all classes seem to be well balanced for group-based PvP. Oh, and yes, you can fly anywhere in the Abyss, and yes, it does create a 3D battlefield. Staying far above your prey can allow for easy ambushes, and spreading your wings can make for fast escapes, and easier kiting. There is no question in my mind that Aion is very much focused on creating fun PvP.
However, if PvP isn't your thing, you can actually skip it. You don't have to go into the Abyss, and even if you do, you can try your best to avoid other players, or to just run instances.
Outside of combat and PvPvE, Aion provides you with the other standard gameplay system found in most MMOs: crafting. Crafting is a very straight forward affair in Aion. You have a recipe, you get the materials you need for the recipe, and you click "Craft". Or rather, you gather enough to make multiple versions of an item and click 'Craft All' which automatically queues up everything for you. There's nothing special about it, but it does work. However, I warn all future crafters: you'll take some serious losses while crafting. I've known people to make money with alchemy, but beyond that, unless you're very clever, crafting will be more of a time and money sink than a way to pay the bills.
Polish and Service
Aion is probably one of the most polished freshly-released MMOs I've ever seen - not surprising, given that it actually was released a year ago in Korean and China - but it does mean that the game is stable, fairly well balanced, and it contains plenty of content. It may not do an awful lot new, but what it does, it does very well.
That said, it isn't a perfect romp either. While my issues with the game are largely limited, the ones I have are actually directed more toward NCSoft, than Aion itself. It seems like customer support in the game is pretty awful. When I get stuck in the world, the last thing I want to see when submitting a ticket is "Average Wait Time: 4 Hours and 14 Minutes" - not to mention the gold spam in Aion is downright horrific, making some public channels next to unusable. Let us not even talk about how painful the first few days of launch were, with locked down servers and giant queues... but, that has since gone away, so I can't really hold it against the game now.
I have to figure that in, as it is part of the quality of the game. Yet, it is an unfortunate blemish on what is otherwise a very solid, very enjoyable MMO experience. So, even with that said, I am still more than willing to recommend it.