GamerScore - When will the achievements unlock?

When I first heard about and saw gamer score and achievements I was impressed.  Here was a system that not only let you have social bragging rights, but also encouraged you to play the game - perhaps in ways you would have never thought of or ways that add an extra challenge/dose of fun to the experience.  

All  of which are ways for developers to encourage people to play their game - and hopefully keep playing - as well as enhancing the community of XBL.  But I have always thought a little more could be added to the system to achieve the ends mentioned above.

The aspect I have always felt is missing, is that apart from the virtual bragging rights and intangible sense of achievement getting an achievement gets, that there is nothing actually earned from your gamerscore.  Instead of how it is at the moment, why not build gamerscore into a "leveling" system?  With achievements earning you xp and each level unlocking something?  It is not like XBL is lacking things to unlock - avatar clothes/items, themes, gamer pictures or even some MS points.

This is already happening with avatar awards being attached to certain achievements in games, but why not make this part of a larger metagame of using your Xbox?  Perhaps you could even add a few basic system achievements to help new users use the Xbox - the "turn off the xbox using the controller" achievement, the "add mp3's to your HDD" achievement or the "play a game with a custom soundtrack" achievement for example?

Turning gamerscore into a leveling system with concrete (well as concrete as free virtual stuff is) benefits as well as the current warm fuzzes would surely also encourage people to buy more games, as well as spending more time in each game that they purchase.   

So why hasn't it been done?  Too expensive?  Anyone got any insights?