Drugs. Was once heavily involved in my school's Just Say No program. Now I'm all "hey hey hey, smoke weed every day."
God. Taught from a young age that the bible was real and true, and then you look at it objectively and... yeah, no. Jesus was a great man, but the Son of God? Born of a Virgin? Nope. Not buying.
Abortion. I'm still against abortion (would never ask or tell a girlfriend to have one), but I'm also in favor of a woman making her own healthcare decisions. Can't see how government minimalists feel they should have a say over someone's body.
The Republican Party. Grew up in a Republican household, with conservative values. The neocon movement completely changed my mind on this.
Nobody puts Jay-bee in the corner.