What I think I've learned recently:
1. Making a character likable/relatable does not necessarily mean making readers feel sorry for her. This tactic often creates the opposite effect through sentimentality, overt authorial pleas for sympathy, general mushiness, doormat-ism, etc.
2. Evil is different from mean. Evil is interesting and requires intelligence. Mean is knee-jerk (h/t CZ).
3. If you are writing for publication, your book is not 100% yours.
4. Altruistic punishment (a.k.a. comeuppance) is something readers really want in fiction. Perhaps especially because it doesn't seem to happen that often in life.
5. I hate/love revising.