Perhaps this is no longer true?
Regardless, the fact the downvoting is just as easy as upvoting (and has no undo) strikes me as odd.
I suggest downvoting cost the voter something, not just the votee. 1:1 seems simple enough. It would also give greater pause to newer users than veterans. However, an undo (perhaps timed like comment editing) would be more desirable in this case.
Regardless of the original purpose, I think that fomenting conversation should be one of the most prominent ideas behind the whole karma deal. The comments and the conversations are what make HN different (and, of course, demographics, which consequently means that the comments of this given demographic, self-targeted, is what makes HN great), and I know many concur with that statement.