Honestly, most excited about the camera. I'm coming over from a crappy 3G camera, and only one of them, so that's by far and away the biggest change. Coming over from 3G, this is really all exciting though.
What do I think is the most important thing Apple has done though? Siri, by far. Just as Apple is bringing us into a sci fi world of digital paper and Star Trek PADDs with the iPad, Apple is also now bringing us into real human-computer speech interaction. Anybody can memorize a set of phrases that their computer can understand, but Siri is taking things a bit further with actual understanding of meaning, not just the words. This technology will only get better.