I think the “mediocre” vote is really a vote on a post being noise. Instead of just “karma,” one could have four numbers: signal, noise, agree, disagree. You can only vote these numbers up, and you can only vote up one of the 4.
A post would then have two scores: a “signal to noise ratio” and a “agree/disagree” score, which would be a the agrees minus the disagrees. (And actually, the signal to noise ratio would not necessarily be treated as a ratio. Both signal and noise numbers will probably be displayed.)
A vote on agree/disagree would be treated as an implicit upvote on “signal” by the post visibility algorithm.
This would make the karma system harder to game. You can vote “noise” to try and censor a post you disagree with, but then you can’t also disagree with it.
From what have just said, I surmise that 4chan is a actually a well-tended garden. I could well be a well-tended, thoughtfully organized, subtly organized anarchy garden.