User Interface: The high resolution screen is gorgeous. Helvetica has never, ever looked so good on screen. Everything is very fast, very responsive. When you drag something – whether it’s the slider button to unlock the phone, a zoomed-in photograph, or a web page – the drag keeps up with your finger. I haven’t found a single element of the iPhone UI that doesn’t feel super-snappy. The whole thing feels very realistic.
Update: Real-time dragging is such a priority that if the iPhone can’t keep up and render what you’re dragging in real-time, it won’t even try, and you get a checkerboard pattern reminiscent of a transparent Photoshop later until it catches up (typically, an instant later). I.e. iPhone prioritizes drag animation over the rendering of the contents; feel over appearance.