Suppose that a user posts a link-only answer which gets a negative score. How should the users who down-voted that answer be notified, once the answer is edited and changed into a proper answer? If someone flagged the answer as low quality, how should it be handled?
As for the reason why notifying the down-voters, Suppose it stands with a negative score for long time; for future visitors there is a chance to misunderstand the post is not correct or not valid.
I disagree. Votes are too important to make people reluctant to cast them. Notifications like the one you proposed would quickly flood inboxes. We are pretty safe as we are here for relatively short time, but think about Clive or kiamlaluno - after 2 years of their voting there must be hundreds if not thousands posts they have voted on.
Also, I disagree on only notifying downvoters. Assuming content may only get better is too optimistic for me.
Edited questions are bring back to the front page. If it is good now, it will earn it's share of upvotes anyway. So the only thing OP needs to do to get out of question ban is to make his question really good. If downvoter is still active on the site, he will see it. If he is not, nagging him with notifications is pointless.
