F**k gamerscore, achievements and trophies. In my opinion, the existence of these has done massive harm to games that would have been significantly more enjoyable without them. By giving people these little targets ("gamifying" games themselves, a meta-game) it alters the way people approach and play games. You no longer play how you want (or would have in the absence of them), you perform certain actions specifically for them, actions the developer dictates.
These meta-game things can keep one thinking about having to do this action, play that playstyle, find that collectable instead of just having fun and being immersed in a game. Considering that most people will only play a game once, I don't doubt there are many who will look up guides and whatnot to ensure they get the most of these achievements is that playthrough. It takes away from the enjoyment of the game by making us focus on these minor tasks...in many cases requiring grinding for hours to achieve or performing actions, making choices or playing classes we otherwise would not have. How many times has the existence of an achievement in a game influenced or guided your actions, in honesty? Have you intentionally gone out of your way, played a certain way or made a particular in-game choice for one?
A good example of this is in DX:HR (a game I otherwise greatly enjoy). There are "achievements" for completing the game without setting off an alarm, for not killing anyone, for finding every XP codex. This means players are shoehorned into the "developers recommended" playstyle, all the weapons and weapon mods may as well not exist. What happens if an alarm is accidentally set off? That's right, the player reloads, all because of the achievement. In ME1, there are achievements for spending sufficient time with each of the crew members, which means to get them all, we have to play with characters we might not want to, may not suit our playstyle or benefit our Shepard tactically. (I remember rolling an adept, playing as far as the first Mako mission then doing the "glitch" by casting singularity on the Mako to get the achievement. This was years ago when I mistakenly gave a damn and in fairness, I game about which I did (and the singularity, in fairness, was the most uber bonus power to unlock for vanguard)).
Yahtzee on this very site summed up the pointlessness of achievements in his WoW: Cataclysm video [http://www.escapistmagazine.com/videos/view/zero-punctuation/2634-World-of-Warcraft-Cataclysm]. (Skip to 3m14s for the relevant part).
Keep gamerscore away from the PC, kthx. IMO there is nothing more liberating and immersive than playing a game without the plague of scores and achievements.