You can up and downvote posts, but why can you only upvote comments? What is the thinking for not allow any sort of downvote on comments (not that I am saying that you should lose or gain rep from votes on comments)?
See extensive discussion on Meta SE. Quoting the Jon Skeet:
If a comment is wrong, respond to it with another comment. That provides a lot more information than a downvote which could mean anything.
If you want to debate this, take it to Meta SE.