Let me tell you about my guild. I'm really happy with it.
We are a small group: we have, right now, seven folks who put in some significant play time each day and an eighth who plays as his work schedule allows. (I'm told that growing a small business from "just me" to "me plus a couple of employees" is a whole lot of work. I believe it.) We span a couple of decades in age and the mainland US's three time zones.
One of the great pleasures of my life is making connections between people I like who hadn't known each other. Red Harvest is becoming sort of a core sample of my years playing WoW, with people I've known anywhere from a few months to several years, plus at least one I knew for a long time before WoW came along. And people seem to be enjoying each others' company.
What we all share...well, one of the things we all share is an appreciation for the fragility of camaraderie. We know, for instance, not to invite in friends of friends without some prior screening. We know that it's very hard to make any decisions at all on a basis of equal footing with more than, oh, a dozen or so members. We know that we're fortunate in being able to foist the work of raid organization off onto the raid alliance, leaving us free to be social and thinking about 5- and 10-person challenges only.
The name is a Dashiell Hammett allusion because that's the way I roll. :)
We operate in a way that is, I think, distinctive to the mature part of a successful game's lifespan. All of us already have at least one level 80 character, and we've done most—not all—of the instances and raids available so far. We know how this stuff works. So now we all feel at leisure to take these characters and do exactly what we want with them at exactly the pace we want. Making progress at it, too.