As I see it, the most common causes for down-voting a question should be:

1. The question is completely incomprehensible, or makes no sense ("Are all prime numbers green?");
2. The question has a trivial answer and really doesn't belong in MathOverflow.

I often see posts (not my own, but there it happens too sometimes) that get down-voted for no apparent reasons: The question is understandable, and it is non-trivial, and I'm sure in many cases the down-voter wouldn't be able to provide an answer. 

How about this: Users can only down-vote a question if

-  at the same time they provide an answer showing why the question is really easy, **or**
-  they can point out where there is an ambiguity, or worse, "real non-sense" in the question.

But they can't down-vote a question any more "just because they don't like it".

(Feel free to down-vote this suggestion :-))