Of the "I wish I knew then what I know now" aspect, the emotional intelligence would be the one area I'd choose. The absolutely stupid and meaningless things that would set me off back then just make me embarrased/ashamed of that behavior now. I'm practically zen-like now compared to then, and I have a much better quality of life for it now. I would stew on things and not let things that bothered me go then, where now I actually laugh at things instead.