Two main examples came into my mind when I was writing that. I used to play a lot of counterstrike on private servers and I played some ultima online as well.
With counter strike, I frequented one server and got to know the players on it a bit. You'd chat a bit after you died but, mostly, I'd just see the same names most of the time. There was usually an admin online and bad behavior (mostly not game related but social behavior) would be banned eventually.
In UO, there was a lot of player interaction where I would actually make friends and aquatintes with people I interacted with a lot. You end up building a community within a community of people you like to play with.
Both of those groups had different ways of filtering bad behavior but, I think the key difference is that they both had a pretty significant social interact component. CS seem semi-accidental but the post death chatting gave it a little something I never got from other fps games.
MMOs have really gone completely away from social gaming. They've streamlined so many processes that actual relationships aren't really that important.