First of all, thank you for Imgur. I feel like i have finally found my family. that being said, every family has a drunken uncle or two, and here they take the form of downvote fairies. It seems that in the mad dash for top comment some people have found that downvoting every comment around them can keep theirs high on the list and visible. Two possible solutions that i have seen:
1) Make each downvote "cost" points from your accumulated reserve (though some people might try to get around this by un-downvoting comments after theirs is clearly a runaway lead)
2) Make total upvotes/downvotes given from each user visible on their profile. that would make it easy to differentiate between a person who occasionally downvotes a bad idea from someone who downvotes competitively.
That being said, I love Imgur. These are just suggestions. keep doin' what you are doin'.