The innocence, man...not having to worry about whether girls like you. Not having to drive anywhere. Not having to take responsibility for anything. Actually being the best in school.
But there are a lot of things I DON'T miss about being a kid. So it's a trade-off. To all kids on this site, you only get to be a kid once, so enjoy it while you can.
Also, I should probably note that I'm still 16. It still feels a LOT different than 12.