World of Warcraft
See from the moment I heard about a Star Wars MMO, years ago. I predicted it wouldn't work because of the Jedi problem. Because I had already experienced the Jedi problem in the tabletop Star Wars RPG. Simply stated everyone wants to be a Jedi but they can't. As you can see it's even worse in an MMO with thousands of players than it is in an RPG with 6. And so you have to limit their availability and whichever metric you choose as a limiter anyone who scores low on that metric (whether it be time, money or cleverness) is going to be pissed that they can't play a Jedi. Now I confess that I never played Galaxies, but from everything I read exactly what I predicted came to pass, and Galaxies, after an initial surge has basically declined in membership ever since.
So now WoW is introducing the first legendary class. The Death Knight and though I think they're not in quite the dire straights that Galaxies was they still may have painted themselves into a corner. See if they make it the same as every other class then people will be mad, "It's a Legendary Class for crying out loud! I had to do a really hard quest!" But if it's not the same then everyone will want to play it and that will break the game because no one will every want to raid or party with ALL Death Knights, nor will the verisimilitude of the environment hold up very well (and it's weak already) if no one plays anything but DKs at the highest level. Anyway we'll have to see what happens, but virtual worlds are a strange creatures.
Part of the reason I was thinking about all this was that there was an article in The Economist about people studding a disease which had been released in WoW, and there were a lot of surprises. More people than they thought were heroic, a lot of people died of curiosity, and then there were people who attempted to intentionally spread the disease. Now obviously when all that's on the line is virtual death people are going to behave differently than when they could actually die, but apparently, for the scientists, the results were close enough that they want Blizzard to do it again, and I guess, according to the article, they've tentatively agreed.
I get bored to easily to be addicted