Actually, I once managed to write a character being practically castrated (got his member sliced off). So... technically I am capable of writing that.
Too bad the story (Puppet Master, Soul Calibur) got raptured in the great AFF change some time ago.
Hmm... What wouldn't I write? I've yet to explore much of it. Actually I'm curious about a lot of the extreme stuff and probably I'll try writing, say, snuff or scat or bestiality. If you had only written a few stories, you can't actually say where your limits are.