I miss thunderstorms and the cost of living, particularly to either rent or buy a property.
I was very ready to leave Iowa after living in Des Moines for a couple years post-college. There were some things I liked, but I was ready for a big city experience. Turns out, I really like it and don't see a scenario where I ever move back to Iowa.
The "nice" thing is a little more perception that reality. I found people in Seattle, Chicago, and Los Angeles to be on par, more or less. I can't say the same for San Diego, the people there suck. Most of the perceived "rudeness" from city people is just a function of constantly being around others. You tend to just ignore other people. At the same time, a lot of people not used to being in a big city are oblivious to others around them - walking slowly in a group on crowded sidewalks or stopping suddenly, stopping two steps off the top of a packed escalator, things like that. If there's one thing that's going to get you some attitude, it's getting in someone else's way or generally gumming up the works by not paying attention.