There are two things that really keep me interested in a game beyond the core story and fighty stuff. Certainly, these are important in their own right, but there needs to be more to really maintain my interest.
One is crafting systems. I can spend hours crafting and not doing anything else, if the system really allows it. Aion was best for this to date, letting me craft and level while not actually having to go out and obtain materials. I'll get into the crafting system for Guild Wars 2 at a later time, bit it is interesting, and I enjoy it.
Second, and in the end most important, is guilds. Not that the system the game uses for guilds is a vital part of the experience (though having an interesting one makes being in a guild more fun), but really the fact that I'm in one with my friends. Of course, different games have different systems for how guilds work. Since I'm very interested in Guild Wars 2, and am planning on dragging many of my friends and guildmates into it, I figured I'd investigate how the guild system works.