1. Badly paced story. A lot of games are guilty of this, because they try to stretch what's essentially a two hour story over eight hours of game. Games aren't films. Stop writing them like they are, and figure out ways around this problem.
2. Random gameplay gimmicks. If I've been shooting bad guys on foot for several hours and enjoying it, please don't surprise me with an awful vehicle section. They're not enjoyable, and break the flow of the game. I'm looking at you Half-Life 2 and Gears Of War. Halo, you managed to do it well. Have a gold star.
3. Celebrity voice actors. This has to stop. Again, games aren't movies. There are many talented voice actors out there such as Jennifer Hale, without having to resort to using celebrities. Honestly, I couldn't appreciate San Andreas properly, because as good as his voice acting was, the main villain was Samuel L Jackson, not the character.
4. Bad voice actors. Sort of a similar deal. They break immersion and can grind a perfectly good plot to a halt.
5. Cliffhanger endings. These are bullshit. I pay £40 for a complete experience, not half of one. Most plot threads should be tied up by the end, with maybe one or two left open for sequels. Speaking of which...
6. Unnecessary sequels. So, the hero has fought the evil empire, killed the emperor, restored peace to the nation and got the girl. The story's over. But the game sold well, so here comes the sequel where an even eviller empire comes out of nowhere to attack, and kills the hero's girlfriend etc. Please stop with this. If I'm impressed with your first game, I'll likely look into any new IP you release. There's no need to crank out a sequel.
7. Not releasing a demo. I understand that sometimes it's just not possible to release one, but a demo can mean the difference between me throwing down my £40 or not. I wouldn't have bought Split/Second before playing the demo. Or Darksiders, or Splinter Cell Conviction. But Alpha Protocol didn't have a demo, and I'd rather not take the risk.
8. Making the main quest the least interesting aspect. I'm looking at you Oblivion. I guarantee more people remember the Dark Brotherhood quests than the actual story.
9. Don't make the game linear when the world needs to be explored. Final Fantasy XIII. Corridorlicious.
10. Don't make the game a sandbox when it's not required. Far Cry 2. Guard posts everywhere.
11. Don't make me sit out of the best parts. How cool were those MGS4 cutscenes? Characters jumping everywhere, having badass fight sequences in slow motion, pulling crazy moves. How much of that could I do in the game itself? Fuck all. Same happened with Uncharted 2. I felt like I was privy to a load of cool scripted sequences, without actually getting to fully interact with any of them. If I want a cinematic experience, I'll watch a film.
I think that's all for now. I'll chime back in later if I think of more.