1) Reliable (bug free) features (clear phone calls, one touch speaker phone, keypad lock, calendar/scheduling and phonebook with outlook sync, alarm, 3MP or greater camera, radio)
2) Light-weight and perhaps not too large either; need to fit into a trouser pocket fairly discretely.
3) Preferable if touch screen (but then it must reliable and accurate)
4) Long battery life.
5) Good for heavy use.
6) Not insanely expensive.
Other ideal features to have would be the ability to repel water, resistant to people sitting on it and voice recognition software where I can compose and send sms purely via voice (without having to touch the keypad).