My kryptonite? Onomatopoeia ... or rather the lack there of. I might be the worst phonetic speller in the known universe. And by worst I mean I do it all the time. I do many things by ear and tone that most would consider to be a blessing, like sing and play musical instruments. Unfortunately, I also spell by ear. Not a big thing, but a REALLY freaking annoying thing.
That's not to say I do all of the other writing things well, I just think spelling is one of my greatest foes. What's your thing? Do you rip your dialog straight from bad soap operas? Does your computer screen have more depth than your characters? Let's vent! :-)