I would massively appreciate if, instead of people just downvoting and leaving, they actually send criticism. Perhaps, there can be some feature, where if a downvote is sent, people can send critique? It can get a bit annoying for me if people just down your save, send no advice on improvement, and leave. Sure; the comments section can send criticism, but almost never is it used for sending critique, and quite oftenly isn't even advice.
I believe critique is a thing that certain user's do, but not to rain on your parade I don't think this feature is needed. This is a double edge sword argument. This would lead to people who make spammy saves getting FP. Plus there is criticism in the comments. One could argue this about the up vote button, and in my opinion this would increase vote farming (asking for votes).
So "if" this was implemented and a critique was needed before down voting and you didn't have valid reason. Saves would just reach FP ensuring vote farming. Like I said it's a double edge argument, meaning this could go on forever.
Perhaps, maybe, it could be required? Sure; nonsense comments will be around, but it will at least potentially increase the amount of criticism users recieve by a bit. I also wonder; how would this possibly increase vote farming?
The problem is that requiring a downvote feedback response would leave the user vulnerable to mass downvoting as a retaliatory tactic. There's a rule in most voting systems, that you should not be able uncover the identity of the voter to the public because it can lead to cohesion and other retaliation
Positive votes would lure beginner's to ask for votes, especially if they don't get any votes. This already happens. "Please vote" so add that with what you proposed. "You can only down vote if critique the save." So if the save is a copy I can't down vote or there's not a legit reason? Hater's just fuel my drive to draw or work better.
Yeah, unfortunately systems like this will only affect people who already behave well, and not the people who downvote for no reason without providing useful criticism. The best thing you can do is probably put something like "Feedback is appreciated" in the description or save somewhere. Personally I always point out flaws with mechanisms, but I don't usually critique artwork unless people ask for it.
As for dealing with trolls- try to keep in mind that unless you've stolen a save, or made a spam save, or anything like that, downvotes without any comment are pretty meaningless. If you make something good with lots of likes but a few dislikes, I'd say try to ignore the dislikes unless they provide a reason