Avatar, something even current gen PCs are incapable of.
AMD's director of ISV relationships Neal Robison gushed in the magazine that gamers have a lot to excited about. According to Robison, the Xbox 720 would be not only capable of Avatar-level graphics, but advanced A.I. and physics to the degree that a Grand Theft Auto-style open world game would be able to give every pedestrian and character an individual AI personalty and reaction, rather than the one-AI mob mentality of current games.
AMD is the current card manufacterer for the Xbox 360, and while they haven't been confirmed for the same role in the Xbox 720, it's likely that they will be.