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.