I'd like to see a feature where by if I down-vote an answer or a question which is then subsequently improved by its author, that I be notified of this so I may reconsider whether or not my vote remains appropriate.
This increases the 'reward' for posters who correct their mistakes promptly. Restraining it only to recent posts prevents there from being too much noise (since you can only vote 40x in a day, if you kept it to a couple of hours then there'd be a pretty low limit on how bad it could get)