I love that DVing costs reputation b/c it keeps the trolls at bay
Trolling isn't a problem on SO, is it?
but I also get really annoyed when I see a post with 2 answers, one of which is obviously incorrect, that both have a score of 0. I would prefer that the bad answer have a -1!
Why won't you give the correct answer +1 instead?
Personally, I think that comment explaining why answer is bad will help community a lot more than downvote. A lot, lot more. It will help 'answerer' to realize his mistake instead of alienating him for no apparent reason (the reason why answer is bad may not be obvious to everybody).
edit
Richard JP Le Guen has really summed up my philosophy about downvoting: "Reward the good; ignore the bad and only punish the terrible."