Content-aware fill is a big improvement, and also stitching/automerge is much better in CS5 than in earlier versions, and I suspect it will continue to improve.
64-bit processing is also worthy of note. Of course the machines have gotten faster and I have more RAM than I used to, and I haven't bothered to compare processes in my old CS2 to CS5 on my newest computer, and all of these factors are interdependent, but things that seemed to take more than 20 minutes are taking less than a minute now.
There are also other reasons just to keep everything up to date. Recently I was working on some print advertising that was simple enough to lay out in Photoshop, and we've finally settled on some house fonts for our office, and for whatever reason, it was troublesome to install the Open Type fonts in Vista on my older laptop or in XP on my office laptop, but with Win7Pro/64, CS5 in 64 bit mode, everything worked smoothly.