* Overall interface rocks! Who knew a cell phone could be fun to use? About 90% of the phone is completely intuitive. I glanced at the manual for about 30 seconds and was able to figure everything out. That's a feat of user interface design!
* Maps. Who knew you could get so dependent on a phone mapping program in a matter of a week? I'm in New York for the summer and the iPhone mapping feature has been awesome in locating restaurants, stores, etc. This weekend I'm using it for driving directions. Just great all around.
* Web browsing. I switched from a Treo to the iPhone and I hated, hated, hated trying to surf the web on the Treo. The iPhone's hi-res screen combined with Safari makes web surfing a joy, especially when on a wi-fi network.
* Email is awesome. The interface for checking Gmail is beautiful!
* Photos. The entire interface for browsing and zooming in on photos is amazing. Can't say I use it that much, but it's nice to show off. :-)
* No task lists! I had become entirely dependent on the Treo's task list feature. It keeps bugging you to do something until you have actually got it done. The iPhone has Apple's iCal program, but no efficient way of adding tasks. I've started adding tasks as appointments in the calendar, but this takes awhile and there's no way to dismiss a reminder and have it remind you again later, like there is on the Treo. I definitely hope Apple adds this functionality to the next version of the iPhone.
* Keyboard. There's been endless discussion about the keyboard or lack thereof on the iPhone. I find the on screen keyboard very hard to use, at least so far. I had become pretty adept at typing with two thumbs on the Treo. This seems to be virtually impossible on the iPhone. I have to hold the phone in one hand anf then tap the keyboard with the index finger of my other hand. Not very efficient. Even when using my index finger I find that I type the wrong keys on a regular basis. Hopefully this will get better with time, but so far it's pretty annoying. One thing that Apple could do on the next release is allow the keyboard to swivel horizontally when you turn the iPhone on its side.
* The AT&T network. Painfully slow. I was already on Cingular, so I knew how slow it was going in to this, but since the iPhone makes it so much easier to surf and check email I find that I'm using the network much more now.
So, overall, would I buy the iPhone again? Yes, I think I would. But I tend not to use the phone too heavily for business applications. If I was depending on the phone to type the majority of my email or keep up with tasks, I would have stayed with the Treo.