If I recall correctly, you must complete a stage to give it a "fun" rating. That or things have changed since that blog post.
Actually I don't know. If you need to complete the stage all the hard stages will have perfect ratings since only people who enjoyed them will rate them.
On the flipside, easy but fun stages could get down voted by just about anyone.
Ah well, no use worrying about it, I'm sure in the end trolls won't have too big of an effect.