It's notable that his fanbase includes people who've never even seen a single episode of Supernatural but know of him through his Twitter account, various memes, and his charity.
Ensemble Darkhorse: He originally signed on to play the angel Castiel for only six episodes. Here we are two years later, and Misha Collins gets top billing after Jensen and Jared in the episodes Castiel shows up in.
Jerk with a Heart of Gold: His online/joking persona is that of a ruthless egomaniac with dreams of world domination, but he kind of ruins it by doing things like running a charity website, cheerfully signing autographs and doing photos for hours on end at conventions and sending fans personalized notes and other 'rewards' when they obey his lordly commands. As a result, most of us will pretty much do anything he asks.Awww.
Manipulative Bastard: See Jerk with a Heart of Gold. Now think about if he only does all the Nice Guy stuff (charity, gifts to fans, autographs, etc.) so that when he does try to take over the world, people would actually want him to! After all, it's easier to conquer the world when its inhabitants love/like you than fear or hate/dislike you.