Achievements & Trophies

Cool, so a sense of pride and dedication can be shown through achievements and trophies!

Also there are items that Xbox Live will grant you for your avatar when you reach certain milestones.
There are bonuses to your Xbox Live rewards also.

No, not from game rewards but that is another good reason. A friend of mine has got a ridiculous gamerscore (I can't remember the number but if anyone cares I will go and look it up) and he has a shirt for his avatar he was awarded for it.
You have to sign up for Xbox Live Rewards when your a gold member and you get bonus ms points for achieving certain things and one of those is when you hit milestones on your total gamerscore.

[Michaeljohn]

although i doubt that's enough nowadays..