Vanity-type incentives are likely the best ones to provoke people to do something. However, I wouldn't necessarily think of a rank as that would potentially add some trust issue-type problems, of people thinking that they're more reliable than others.
I disagree with this. There is no zero reason for the (e.g.) contributor rank to make people think that "they're more reliable than others". Similarly, being a donator or VIP shouldn't cause that either. IMHO the best way to proceed is with something like a rank.
Would an icon work?
So you're thinking that there are people who would report thousands of posts just to receive a small icon? I don't even notice those anymore.