Better reflections. In the gameplay video the ground is wet but it doesnt realy reflect the environment well. Since reflective surfaces are going to be so common then such an upgrade would make the game look amazing.
Maybe dx11 tesselation? Apparently all they need to do is whine to ati/nvidia and get it put in the game with the only cost being a logo in the startup sequence. In a large game id expect some lower polly models to be around to save disk space. Maybe one or two hieghtmaps could be used to make some models (faces of the aliens for instance) have unbelievable detail.
Perhaps the pc/ps3 version could have some better textures too seeing as how they have more room in them than the xbox.
