Bummer, I'm exposed.May also smoke out the 'fantom downvoter'
Having to explain and comment on your vote is also a bad idea and demotivating. I can certainly live with just positive votes, it's the best option discussed so far.
What I would really like to see is the actual number of upvotes and downvotes, not just the balance. A request receiving two upvotes and nothing else is probably ... pretty lame. Another request receiving 74 upvotes and 72 downvotes is obviously controversial, but still somewhat of a hot topic. It might make sense to dig into this a bit deeper. Maybe it just needs a minor change to the original idea to gain acceptance?
If this isn't possible (or not wanted by a majority): Go with positive votes, only.