It's time for something new and while the current Gamerscore / achievement system has worked well for the Xbox 360 it lacks some of the modern intricacies that developers want for their new games and there is definitely room for improvement.
The old system has been overhauled by Microsoft in preparation for the launch of their next-gen console, the Xbox One. It is said that you will be able to add new achievements to titles every week after release, a system that developers can use to keep people playing their games and personally I think this is great because anything that adds replay value to a title just adds to its value for money.
It's obviously a system that could quickly get out of hand though so Microsoft is going to be limiting the monthly total of achievement scores via a monthly cap, much in the same way a game launch has no more than 1000 Gamerscore at the moment, with set amounts for each DLC. It could also work that instead of having all 1000 points available from the launch; the game could in fact launch with 5-10 achievements (my example) and add more on a weekly basis to keep those wanting the score coming back.
There will no doubt be new ways to earn achievements and rumour has it they could be system wide, not just gaming based and it will be interesting to see what Microsoft comes up with, for better or for worse.