Four things Mitchell lived to regret and one thing he didn'tthe 4 things he regrets;
dating amy vandenberg. to be honest he doesn't regret the relationship, but the fact that he couldn't give her the attention she deserved.
not studying the sodan culture. he's no daniel jackson, but he feels that someone should somehow preserve something of them now that they're all but gone.
he regrets saying to teal'c that he thought star trek was better than star wars. not only was mitchell made to watch the original trilogy, but he was treated to a lecture on how the series has characters & plot points mentioned in the book the hero with a thousand faces by joseph campbell.
mitchell regrets not buying stock in yahoo when a friend of a friend suggested it.
the 1 he doesn't;
getting SG-1 back together. there are times he questions his sanity over doing it, but ultimately he has no regrets.