I came across this question recently and asked the OP to explain some notation (see my comment below the question). As the OP was addressing my concerns, there was suddenly a down-vote (along with the resulting $-1$). Now, this is awful for at least two reasons:

1. there is absolutely no feedback for the OP as to why a fellow MO-user had a negative perception of his question, and
2. it now looks like I got annoyed by unexplained terms and down-voted even as this poor guy was addressing my concerns and defining his terms.

So, here is my question:

How can we encourage people who down-vote questions -- even bad, unsuitable questions -- to either leave a comment explaining themselves or to up-vote an explanatory comment that is already present?

This type of hit-and-run is really bad for our site and the way it is perceived by those not already using it regularly. When there are down-votes -- but not votes to close -- it is likely that low-reputation users are responsible. There is at least some accountability for voting to close (since your name is revealed upon closure), but almost no accountability for a random down-vote. We really should work on creating a better deterrent to this type of behavior.