My absolute fav is the fact that when you flip the iPhone it flips the picture/video/media also, think its called the sensor flip thingy....lol....its wayyyyyy tooooooo awweesome!!!
My favorite part of the iphone has to be the touchscreen. I have always thought touchscreens were cool and now that you can use it with a phone and web browser its even better.
I actually enjoy the whole Locking Mechanism to tell you the truth, The whole ability to unlock with just a slide helps ease the constant locking and unlocking tasks.
Well... I haven't got an iPhone... yet, but from what I've seen, my favorite iPhone feature is the UI (User Interface) and the way you can interact with it.
I personally think the internet features and google maps look the most appealing.. in particular on something phone/mp3 player functionality.. now it can replace most uses of GPS too.