I just watched the Launch promo for CS5 and I'm not blown away, but they've definitely got some 'wow' factors in some of the improvements and tools. Off the top of my head things that grabbed me were:
Mercury Playback Engine (Premiere Pro)
Puppet Pins, Background removal/replacement tools, brush mixing (Photoshop)
Auto Rotoscoping, RAM Preview capable of fully using available RAM (AFX)
Codeless creation and importation of designs from Suite Programs into Flash and Dreamweaver.
Looks like it will be worth the $900 bucks to upgrade from CS3! BTW, you can catch the promo yourselves at adobe.com. Let me know your thoughts on this.