I can't badmouth the RMAH, I ended up thinking Diablo 3 was pretty mediocre overall, so after I got bored with it in the first month I just botted a shitload of money, sold it on the RMAH, and made about $300 before I got banned. D3 was the first game that I ever made a profit playing, it bought me a new video card
My vote for biggest fuckup was WoW reducing max raid sizes from 40 down to 20. I had a raiding guild from EQ, shrinking it down to 40 was a little rough at the release of WoW, we lost a few good people along the way, and then the shrink down to 20 just pretty much destroyed the guild. No matter what you do, you can't get 2 equivalent 20-man raids out of a 40 person group. Bickering & fighting over who got to be in the "good" group. Shit almost gave me an ulcer, putting together raid groups every night with everyone bitching at me once they found out they weren't in the "good" group, whatever they perceived that to be. You can't make a fundamental change like that mid-game, after it's already set an established precedent for years.
Basically it led to people leaving, the guild shrinking, less people to socialize/group on non-raid nights, and people stopped caring about playing the game. Lame. That, combined with the disappointment that was D3, dropped Blizzard out of my "buy anything they make" category of developers.