This could be done under something close to the current mechanism by using reduced vote power (indeed I already do this sometimes when voting for content that is good but not great). I think that's pretty much what it means for a whale to upvote something but still believe it is really only good enough to rate slightly better than average. A full power vote would also tend to give such content a large reward, which makes little sense.
There are some other interesting aspects to this, and I may write a separate post if I have time later.