I do not get many replies to my blog post, but I do a lot of replying and comment on comment streams from other people's blogs. The ones I reply to and/or upvote are the ones that add to the conversation.

Why would I upvote a comment that said "nice post now look at what I wrote about bee pee", when the entire blog/post and comments before are about how to find polaris and align your telescope for a viewing of the Rings of Saturn.

Like @mehdibca said, the comment needs to be relevant.