On a personal level, it would have to be the announcement of Brothers in Arms: Furious 4.
Fuck you, Randy Pitchford. Fuck you, Gearbox. Fuck you, Ubisoft. Way to shit all over the hopes of fans of the series. You've managed to replace what was respect with unadulterated loathing in just over three minutes. That's damn impressive, in a dickhead sort of way. I hope you're happy, dickheads.
I usually don't resort to such language, but I feel it addresses my dissatisfaction in the most adequate and direct way possible (plus I'm still mad about it).
In general though, it would probably be the "always online" DRM. If I bought a single player game for the PC, the last thing I want is to be punished if my roommate accidentally kicks out the modem chord.