So far, photo editing
and Safari Reader appear to be the only missing features.
As a side note, if you upgrade your iPhone to iOS 5 and are unhappy with the stability (or, like me, you realize that your iPhone isn’t registered as a developer device anymore) you should be able to downgrade without any problems. At least on the 3GS, just boot into DFU mode, click restore in iTunes, and within 30 minutes your iPhone will be back to 4.3.3. No SHSH blobs, no iRecovery, just a simple DFU restore.
Update 6/10/11: Safari Reader is supported on the iPhone 3GS.