Maturity and intelligence.
The courage to see everything as objectively as possible, which in turn means not being egoistic.
If theres one thing I hate, its people who pat themselves on the back, endlessly, for whatever reason.
My nation is the best one evarrr!!!111 My religion is the only true one!!!1oneoneone
My music, etc etc etc.
People who are so full of it, they don't even realize it. I think you aren't an adult until you realize that you are full of crap when it comes to certain issues, as is everyone else.
Once you realize that, you become an adult about it, and can control it, sort of.