I saw this question on X and thought it would yield some good answers.
One that's near and dear to me: being a founder. I love it but I'm more achievement oriented than pleasure oriented and don't mind all the pain it inflicts (relatively). Most people imagine being a founder very shallowly, even people actively trying to be founders. They believe that X or Y is the hard part, and if they can accomplish X or Y it'll be easy, but X or Y is one level of an infinite level game, and each level is different and hard in its own way.