I think a lot of those are totally useless. It's just to brag about your absence of social life on a social networking ^^ /sarcasm
But I do think a lot of those are just nonsense, just some random elements thrown awat at your face, with no further reward. I prefer achievments like 'beat all in the game' than 'go find 100 cows' or 'wear outfits'
My favorite rewarding system was in Rogue Squadron 2 and 3, where the completion of missions (taking into account accuracy, time, live lost, ennemy destroyed) gave you special medals, which gave you some points to actually unlock missions. That really helped improve the lifespan of the game, and added a difficulty level in the game that was really rewarding.
On the contrary, I play Team fortress 2, and the achievments are so dumb that you have actually special achievment servers. I think it's useless imho. Also, I was just looking at the achievments of Mass Effect 2, and really found them totally useless. Some achievments are actually some game keypoints, like recruiting X or using X gate. So you get achievments by completing the game, great deal !
