Microsoft has just revealed that they have reinvented the wheel where Achievements are concerned on the Xbox One. The Xbox One will use a new "dynamic and changing" achievement system. Xbox One's new dynamic achievements system will follow players and sync via the cloud.
The new Xbox One achievement system also gives games developers and publishers the tools to add achievements without the need of DLC. This means that Achievements on the Xbox One can be added in at any time allowing new achievements for things like events or challenges. Video clips of the moments before you unlock an achievement will be automatically recorded using the built in DVR and stored via the cloud so you dont use up your new 500gb HDD.