A lot of games have included randomly generated content. Action RPGs like Diablo and Torchlight have randomly generated levels. Strategy games like Civilization and Master of Orion have randomly generated worlds/galaxies. Yet we rarely see randomly generated content in MMORPGs. Anarchy Online had it, City of Heroes had it (sort of), and I think the occasional Asian import sports it as well. While I certainly wouldn't want it as the cornerstone of an MMORPG's content, it seems like having some randomly generated dungeons in a game might help spice things up a bit and provide some more variety, especially at the endgame. It would need to be done very carefully, and not be the only option for players at any point in the game. Random dungeons should sit alongside the carefully crafted scripted dungeons so that players always have a choice - scripted with story or random with surprises.
Imagine using a dungeon-finder like tool to find a group and then queue for a truly random dungeon. Rather than picking a specific dungeon you'd simply select a difficulty and let the game do the rest. The dungeon would be generated from a single themed group of art assets, populated with random groups of trash mobs, and sprinkled with random bosses. There's no reason why the bosses would need to be basic unscripted fights - as long as the scripting was independent of the environment it wouldn't really matter. You could have dozens or hundreds of bosses in a "pool" that could be drawn from. Undoubtedly every boss would eventually be documented on the internet, but at least you'd go into a dungeon with no idea what you were going to face until you came across it. The dungeons would require more thought and exploration, as you can't memorize the layout of an instance that didn't exist until you stepped into it. I think the hardest part would be making sure the randomly generated environment was contiguous and couldn't result in dungeons that can't be beaten (final boss is in a room that can't be reached, etc.). Surely by now the programming skill and processing power exist to do that though.
Certainly there are risks to randomly generated content. In my current game of Space Pirates and Zombies (SPAZ) every system I've come across has had items for sale ONLY from the faction that hates me, with no missions in the system to improve my reputation. There are ways around it, but they're painful and have turned the game into a bit of a slog. I can remember plenty of missions in Anarchy Online that couldn't be finished because the mob/item/whatever that I was after never spawned, and missions in City of Heroes that were almost impossible to beat solo due to the combination of mobs that were randomly generated being powerfully synergistic to the point of invulnerability.
Despite the risks, I think a well programmed random dungeon generator could be a boon to a certain portion of the playerbase in games like World of Warcraft and Rift where a lot of time is spent at the endgame running the same dungeons over and over and over. Granted, there is a distinct portion of the playerbase that only wants to race through a dungeon as quickly as possible to get the carrot at the end and for them this idea would be nothing short of repugnant. But for those players that want to actually play their game of choice and not have everything mapped out for them ahead of time, it seems like random dungeon generation could be an interesting addition.