I love the custom vibrations for contacts, and that you don't really have limit on how many you can save. Multi-touch gestures on the iPad are also irreplaceable and impressive. I think it's very cool how each device has its own features that set it apart from others. I have noticed though that Apple hasn't advertised new multi-touch gestures for iPad very much, if at all. I do wish the iPhone had similar gestures to keep from hitting the home button so often.
Email has been really fixed up, of course Apple advertised that since iOS 5 debuted. I do love that if I check my email on one device, it automatically updates all my devices.
Apple is just making this too much fun.