How about giving users two upvote buttons. The second one appears X seconds after the first one has been hit. Because really great comments, I've noticed, often provoke first: yeah, good. And then, later: wow, that was really really good. I wish I could upvote it again. (the second upvote can have a different meaning)
Google Hotpot does something like this, limiting the number of Really Great votes you can make with unlimited +1's.