Having just dowloaded the trial of CS4, the first thing that made an impression on me was the new interface. To me, it has a more "Lightroom-ish" feel, no doubt influenced by Lightroom's success. While not as pleasing as the dark background in LR, it still has a more user-friendly feel to it.

As for performance, on my machine, speed seems to be dramatically improved, and it is much better at handling large files. That's one area where PS is far Superior to LR. Many of my images are too big to be imported into LR 1.4 (hopefully 2.1 is a little more accommodating), so I'm forced to work in PS in such cases. While I still prefer LR for my primary workflow, it's nice to see CS4 moving in the right direction.

Lightroom 2.x can go up to 512MP (that's megapixels, not megabytes) or 65,000 pixels. That big enough for ya?

If not, it's because I won a digital scanning back in the lottery and I decide to do a 360-degree panorama a la Stephen Johnson!