"Content-aware fill" and "puppet warp" look interesting if they really work. I'm sure a lot depends on the type of backgrounds. I think new in CS4 was the "content-aware scale". It works to some extent but not impressive. A couple of features I really like with CS4 is the "rotate view tool", the zoom of 3200% (I believe it was previously 1600%) and the flick panning. Photomerge is much improved.