I agree regarding letting skill speak for itself. When the GamerScore idea first crossed my screen I thought it was lame, but honestly... I'm hooked! Acheivements allow you to showcase single-player accomplishments, casual game accomplishments... it gives someone else a clue as to whether you are primarily an FPS gamer or sports or if you are more well-rounded.
Also, there are levels of exclusivity. Often a game will be platform exclusive for x months after it's release, then sold on other systems. GTA is good about that. Yes, there are PS3 exclusives, but even a lot of typically Sony developers are makig 360-exclusive games these days, or making games for the 360 that are typically in the Playstation segment (like anime-based games). Two great examples are the new Square Enix game (Infinite Undiscovery, I believe) which is 360-only, and the new Naruto game for the 360 (may be cross-platform, but I don't think so) which, while not my cup of tea, looks AWESOME and is not a typical Xbox game offering.
An interesting aside:
link
Sure, it's not much, but apparently it's the first time 360 consoles have outsold PS3s in Japan. Nice!