Uncharted 2 is a sequel, so things are being tweaked, things are being added, things are being improved. One area Naughty Dog are working on is enemy AI, particularly how it reacts to Nathan's presence.
They'll now boast peripheral vision, meaning if they catch you out of the corner of their eye, they won't instantly start firing. Because, having only seen you out the corner of their eye, they're not really sure what they saw. So they'll either turn around and look a little harder or pop on over and investigate.
They'll also be more aware, and will make better use of their surrounds. If they see you, then lose you, they'll head to where they saw you last and start searching from there. And if they see you climbing a ledge or jumping over something, they'll do the same thing, meaning combat is going to be a lot more "3D" than the first game.
All well and good, but what we really want to know is: are we shooting at humans this time? Last time we checked (and be assured, we check often), a shirtless Mexican didn't take 17 bullets to the chest to kill. Fix that and all this extra stuff will just be icing on the cake.