Someone once told me that EI sometimes adds 10px to some elements in div tags. Can anyone else tell me what other little quirks there are that I should keep in mind when designing?

IE6 parses whitespace sometimes, so if you have a space between two <a> tags, you will get a gap... yay

You will find that most IE quirks relate to something calles hasLayout. Here's an comprehensive article about it: http://www.satzansatz.de/cssd/onhavinglayout.html