Getting 40 people to do anything together is challenging. Heck, getting even 5 people to agree on where to go to lunch can be challenging! So I find it very impressive the level of coordination that emerges in games like WoW to tackle raid challenges. I've finally gotten to the point of doing some Molten Core raids -- a high level dungeon in WoW that requires 40 people to full off some fairly elaborate tactics to beat some uber bosses. Ok, it's not the toughest dungeon in the game... but it is pretty close.
The experience has a pacing and intensity that is quite different from most other multiplayer experiences. It's a 40-player co-op game but in most cases the ideal strategy is already known from the legions of players that have come before and "solved" each of the encounters. The trick is in executing those strategies with players who haven't done it before and who may not have the ideal gear, class balance, etc. But really, the core challenge is communication. The issues are human issues, not game mechanical ones. Leadership and planning, not hitpoints and spell power. That gives it a human intensity which is definitely an interesting flavor of multiplayer gaming.
I also find it fascinating that there are user mods designed for specific high level bosses in WoW. It makes sense, of course, since organization is so key and computers are very good about things like warning about special attacks that occur on 30 second intervals and the like. But these tools are so far outside the actual in-game structure of the game it almost feels like cheating to me. Blizzard draws a clear line between what they say is valid and what isn't, and these mods (like "CTRaid", the most common high end raid-oriented UI mod) are clearly sanctioned.
Admittedly, at the point where we are fighting the same bosses time and again to "farm" them for their loot, we're well outside the realm of a coherent story or in-game fiction. But a part of me balks at the idea of having to go so wholly outside the game. Social pressures make it a de facto requirement to have these mods to participate at the high end. It's fantastic that Blizzard has such an open UI format that such things are possible... but it certainly is not a decision that comes without cost. Just beyond the issue of being required to download this random third party software to play the game, it must result in a great deal of technical support and security issues for Blizzard. Yet, now that they have upped the ante, it is hard to see how any top tier MMO will be able to ship without that capability.
Some "classic" open-ended PC games have really been eating all my time lately. In addition to the aforementioned WoW raiding, I've been playing a lot of Magic Online as well as GTA: San Andreas. Really, I could probably play either of those games for another year without getting sick of them, and when I get sucked back in my "pile" definitely grows. Fortunately it's a bit of a lean time right now for new releases, but that certainly won't last...