I want a portable computer in its own right; something that doesn't need a tether to a desktop to work. The iPhone is completely dependent on its host system for important things like system updates. This is where I really miss my T-Mobile Sidekick (aka Danger HipTop). Everything it did was OTA (Over The Air); from system updates, to syncing. Heck you could even lose your phone and not worry about your data because it was stored on the web! It didn't do everything perfectly (the online web interface was atrocious) but it got close.
Some features that I'm looking forward to on the Prē are OTA updates, online syncing, and background apps. After using Google calendar quite a bit I've realized that it's much better than Apples iCal program. Google makes it easier to have multiple calendars, share information with others, and generally navigate the system. I also love to use SSH to manage my servers, but the iPhone makes this hard if not impossible because of the lack of background apps. When I switch out of my SSH program to check an email, the iPhone closes the SSH app thus terminating my connection. Not cool.
One thing I wont miss on my iPhone is the music playing function. For one, I already have an iPod that works really well as a music player. But the biggest problem is that the iPhone makes a terrible iPod. The original intent of the iPod was to make it very simple to listen to your music and the iPhone screws that up royally. Its very confusing to control the music playing in the background while at the same time switching between other applications. For some reason I can't seem to do it. And forget about playing online music while doing something else (see the part about background apps). I was really looking forward to Pandora Radio or Last.Fm on my iPhone when it was first announced only to be disappointed.
I'm not saying that the Palm Prē will be the end all be all system for me. Its not even been released yet. But I think it will come close and I am looking forward to giving it a shot.