I haven't lived in Spain but I spent 2 years in southern Europe. I thought I was prepared for the slower pace of things but I really wasn't. The DMV has a bad reputation, but you will be thinking fondly of it if you have to interact with any bureaucratic institution. Also be prepared to be hot as hell in the summer. Most countries aren't big on AC. And learn Spanish (or Catalan depending on where you move) if you don't already know it. I didn't speak the language and got by, but life will be MUCH easier if you do (even a little).
Don't let this scare you off, moving to Europe was the best decision of my life. Make sure you travel around a lot. I personally think all Americans should be forced to spend 1 year abroad, so if you've got the option take advantage of it. Some of Spain is anti-tourist now (ex Barcelona) but I've been to a couple of places and really enjoyed the food and architecture.
