If you know my work, then you know I've clocked a few hours playing online games of the massive variety. Yet in the last three weeks, I've spent more time playing with other people than I've spent in all of those other games combined.
Online games have been trying to get players to group up for years. Players generally don't want to group, because it takes time away from playing. You have to hunt around for internet strangers who are the right level, the right classes, in the right area, and interested in doing the same thing. If someone needs to have a bio break, then everyone in the group needs to stop playing until the group is ready again. When your healer is not doing his job and getting people killed, do you stop and teach him how to play, kick him and sit around doing nothing while you look for a replacement, or charge forward and just put up with it?
(Nitpick shield: No need to shout, "WHAT ABOUT RAIDS?!?" at me in the comments. We're talking about the open-world questing here, which is the "main" part of the game for most players. Remember that not all games are WoW, and not all players are at the level cap.)
The grouping stuff is mostly an annoying time-sink, and none of that it is really "playing the game" in a mechanical sense. It's busywork you have to do in order to get to the game. Grouping up is to playing as arguing over restaurants is to eating. World of Warcraft made great strides in taking these tedious tasks and streamlining them. The WoW dungeon finder is probably the biggest single improvement to group play in years. But it wasn't until I played Guild Wars 2 that I saw what could happen if someone threw the entire system out the window and started over with the assumption that players should play together by default.
The problem with those old games is that the quests and dungeons forced you into groups, but the gameplay mechanics pushed you apart. If you group with other players, you have to share loot with them. If you're in a group, you get less XP per kill, even if you happen to be doing all the work at the moment. If I'm in a group and I help a friend do a quest that I've already done, I get less reward than if I just ran around killing stuff on my own. The game is discouraging me from playing with my own friend!