From the latest gameplay demo (the one with the car chase) which seems to have been on the 360 as far as I can tell (because of the control scheme), it looks like it'll just be a difference in graphics. I'm not too sure, but I think that the PS3/360 releases might be lacking things like weather effects and i'm also assuming that there'll be a difference in AI in one way or another, that's just a guess though. All that seems to be true for now is that the graphics are different, other than that, nothing else.
Currently waiting the release of The Guided Fate Paradox (Fall), Disgaea D2 (Fall), Dragon's Crown (August) and Time and Eternity (Summer)
Remember the trailer where the car crashes and one pedestrian rushes to help the driver out of the car and drags him to the sidewalk all in real-time? It is REALLY tough for me to see that happening in current gen consoles.
Imagine that sort of complex actions being performed and calculated for every human on screen during a firefight on a street when cars are crashing everywhere and cops are taking cover and keeping pedestrians away etc.
Yes there will be graphical drawbacks in terms of weather effects but there just HAS to be a decrease in the number of people on screen at one time because then the people on screen will at least have the resources to make complex decisions and stuff.
I would rather have less people but functional people than more people acting like meatbags.