First I'm really enjoying the iPhone and think it's going to stick with me, so I should start with that.
Second, the biggest noticeable issue with the phone is the battery life. To be honest, battery life was the biggest issue I had with my WinMo phone and I'd hoped to make an improvement moving to the iPhone. No such luck. In fact, the iPhone - even with Wifi disabled - runs only about 60% of the life of my Tilt with about the same usage.
I purchased the Morphie Juice Air with the integrated extra battery. This just gets me through a full day (6am to 10pm). This is a tough pill to swallow.
The third point is the keyboard and editing challenges. I've been using touchscreen keyboards for years and am pretty proficient with them - several folks have already commented on my iPhone keyboard speed. That said, the three biggest deficiencies I've noticed are the lack of "in word" editing, lack of cursor controls and, of course, lack of cut-and-paste.
The first two are obviously an artifact of the lack of precision of the capacitive touchscreen v. the resistive touchscreen technology found on essentially all WimMo devices. The trade off for precision is ease of touch and the non-recessed screen found on iPhones and other capacitive screens. The latter is another of Apple's compromises that should be fixed in OS 3.0. Nonetheless, I am constantly reminded of these deficiencies when doing email or taking notes.
Fourth, Also notably missing is the ability to reorient the keyboard to landscape. This is truly an oversight on Apple's part in the user interface - this is the kind of intuitive feature they are normally way out on front on. It must be some frustration for the Apple designers to be bested by literally all their competition on.
Fifth is the actual function as a phone. In most ways in this regard, the iPhone is fine. But I am experiencing a noticeable drop in coverage and connectivity over my Tilt -which is an AT&T device, too. Many blame the iPhone's deficiencies in this regard on AT&T's network, but my experience seems to indicate it has more to do with the iPhone design itself rather than with the network.
All these quibbles aside, the iPhone remains am intuitive, elegant, smooth and speedy device.