There's a great post on West Karana about how recent MMOs are less social than before. That's something I've also been frustrated with. When I played World of Warcraft, particularly, groups would form very rarely, and people would simply drop out without a word as soon as they got what they needed. The guild I joined lost people as quickly as it replaced them. It wasn't like that when I played FFXI. People would actually take the time to get to know each other. People NEEDED each other to advance in the game, so they worked hard to help each other.
Why are newer MMOs less social? Part of the reason, as Tipa points out, is due to the ease in which a player can solo through the game.
Another reason, in my opinion, has to do with quests. Once upon a time, when all we did was grind mobs, you only needed to find players in your level range in a handful of appropriate zones. Now we have all sorts of additional barriers: we still need to find level appropriate players who want to adventure, but now we also have to ensure they have the same quests as us. That's often more frustrating than its worth.
EQ2 has improved a lot lately by improving the LFG tools and introducing quest sharing to encourage grouping, but it still seems to be much rarer than it used to be. But even with quest sharing, quests (as they are implemented right now) are still problematic because they typically involve multiple steps that take you throughout a zone, and backtracking doesn't reward anyone who has already done the quest. So quests in a sense just divide the playerbase up further, making it harder to group. This drives many players to pursue soloable quests instead, which means they no longer have an incentive to group, which further reduces the available playerbase for those that do want to group.
That doesn't mean I think we should return to the days of old, when we were forced to group and grind mobs. Quests are good in that they keep players moving through the game world instead, experiencing more of the content. But the way they are implemented in MMOs right now is simply broken.
I think a true next-generation game will try to address those issues. Warhammer Online may very well be on the right track with its concept of public quests: this type of quest is available to anyone in the zone at the time. That has the potential to help foster a sense of community giving players a common goal to work together towards. I don't know how they have it implemented, but if they do it right, it could be that revolutionary design change this genre of gaming desperately needs.
Its too bad for me I haven't been impressed at all with what I've seen regarding Warhammer Online's gameplay, nor do I care much for PvP. But they do have some great ideas regarding the quest system that I hope will be stolen by other game companies (*cough* Sony). I think if we shifted the emphasis away from stupid, boring kill and fetch quests towards more public quests, or something that builds off of that idea, we could end putting the multiplayer back in these games.