Trenton sent me a link to his long, but very thoughtful and well-researched article on MMO Community Enhancement. It contains some great proposals, like an automatic "ally meter", which measures how much you interacted with various people, and acts like an improved friends list. Nevertheless I don't think he covered all the potential problems, or that his solutions are valid for every game.
One example is his proposal to let players join multiple guilds. Sounds great on paper. But in World of Warcraft the one thing that would make this difficult to implement is raid IDs / lockout. Even if I wanted to raid with two different guilds, I couldn't, because of raid IDs locking me out of participating to raids with a second guild.
Trenton is right on in his chapter on Crafting Attitudes Through Motivation, that is whether and how people play together depends on their motivation to do so. But his solutions here aren't complete, or not fleshed out enough. He is right to say that in a much more difficult game like Everquest, where you *need* a group to earn experience points, cooperation is stronger than in a game like World of Warcraft, where you can solo all the way up to the level cap. But I don't think that difficulty is the only parameter here: Rewards are at least as important. World of Warcraft could have significantly more groups forming voluntarily, if only grouping would give better rewards than soloing. As it is, you gain more xp per hour while soloing than when in a group, thus you only group for content that is too difficult to solo, like group quests and dungeons. But if you expand on Trenton's solution of "share results", and would make everyone in a group gain full xp for every monster killed, instead of xp divided by number of group members plus a small bonus percentage, then players would want to group much more often.
In any case, Blizzard would do well to read that article. There are a lot of tools in there which they could implement into World of Warcraft (or their next game) and make it better. WoW's guild tools for example are rudimentary at best. But I do think one thing WoW got right is how everyone *wants* to be in a guild, because raid content is very much linked to guilds, the occasional PuG raid notwithstanding. I often felt that I needed my guild a lot less in other games, even if that game had better tools.