I have to strongly disagree on that point. Being able to "counter" your vote opens up very little room for any abuse unless you implement it poorly. If I upvote a node, then I could change my mind and take that one upvote away as if I'd never voted on it, but I'd probably still see the rep and not be able to vote on it again, and it would cost me two of my votes. Similar situation for downvotes (which is really the more likely scenario -- undoing an accidental downvote).

But I wouldn't put such a feature high on the priority list. I'd probably put the "zero vote" above it (whereby I can vote "0" on a node which costs me a vote and lets me see the node's reputation but offers no change in XP nor reputation). I already vote on some nodes that I don't really think derserve an up nor down vote because I'm especially curious about their reputation. /: