My top three smartphone requirements:
1. Right now, whatever smartphone I have must run Windows Mobile. It's not that I don't see the benefits of some of the other platforms, or that I think Window Mobile is the end-all of mobile operating systems, but I am presently tied to a single piece of Windows Mobile software: PocketBible by Laridian. I have eight years of personal notes in my PocketBible database that will be lost unless Laridian devises a way to move them to another platform's Bible software. This has been the single biggest factor keeping me from moving to a Symbian device.
2. It must provide quick access to all critical system controls, either by hardware buttons or Today Screen/Task Bar/Tray icons. I hate digging into nested menus or screens just to get to the Wireless Manager, for example, when a quick press of a button should do it.
3. It must have a useable hardware keyboard. I just can't seem to get comfortable with software thumboards (though I admit that I haven't yet tried the iPhone's). Transcriber is pretty reliable, but it still requires too much back tracking to make minor corrections. Small screen keyboards that require a stylus are just intolerable in any incarnation.