As much as I appreciate entertaining others with my stories, I don't write for anyone else but myself.
I guess the biggest challenge I face is surprising myself when it comes to my characters. If I find I'm liking them too much, I have to cut them down a peg or two. Life isn't safe, so why should my characters be safe?
Creating a completely original character, based on no one in particular, is hard to write for. You dive into these emotionally internalized characteristics and suddenly you begin exposing your own skeletons without even realizing it.