Here are the things I hate:
Silent Protagonists: Now there are a few exceptions to this point, but I'm just going to ignore them for now. Basically, if you're playing as a hero who is mute or cannot talk, you don't seem to care much about him/her. If the story decides not to show you information on the main character, then you'd expect him/her to talk so you know their personalities and such. There are times when you want the hero to talk instead of staying silent for the whole time, like near the beginning of Dishonored where you want the main character to stand up for himself and say he didn't kill the leader of the people but instead he stays silent and then becomes a fugitive.
Client-Side Hit Detection: The opposite of Server-Side Hit Detection. Basically, if your internet connection is garbage then you can easily beat everyone who has a good internet connection. One example is Battlefield 3, which uses that type of hit detection. This means a guy from Brazil or Saudi Arabia can go into a North American server and kill players like no tomorrow which pisses off everyone from North America playing there because they can't hit that player from Brazil or any other country and by the time they do they're dead.
Spawn Protection: Spawn Protection allows players that have just spawned into the match to not take any damage right away. That's good to prevent spawn camping, but bad when the player can kill other players during the spawn protection. Spawn Protection can give players an unfair advantage over players that spawned a long time ago when they can do whatever they want during the protection period.
Horrible Artificial Intelligence (AI): Horrible AI can ruin the singleplayer or co-op of games completely. If the enemy AI sucks, then the game can become too easy. If the friendly AI sucks, then the game can become too hard. If both AI suck, then the game sucks. Homefront had horrible friendly AI. So horrible, that when I saw how the US Army soldiers were fighting in the singleplayer I was like "No wonder North Korea took over America..."