I think achievements were a brilliant idea from MS. Small, but great innovation.
I think a lot of games need to reward some actions in a better way. When I finish a game (you know, BEATING it) on the hardest difficulty, I want at least 2/3 of the achievements. I don't run around and do some stuff just to get achievements.
700 for finishing the game on the hardest level.
200 for some multiplayer stuff
100 extraordinary things to master.
That would make sense.
I think if you finish GTA IV, which is quite a big game in the first place. You only get 200 or 300 gamerscore. Makes no sense.