On John Scalzi's Whatever, a list of ten excellent rules for being a better commenter -- it's certainly stuff that I'll keep in mind the next time I leave a comment somewhere:
1. Do I actually have anything to say? Meaning, does what you post in the comments boil down to anything other than “yes, this,” or “WRONG AGAIN,” or even worse, “who cares”? A comment is not meant to be an upvote, downvote or a “like.” It’s meant to be an addition to, and complementary to (but not necessarily complimentary of) the original post. If your comment is not adding value, you need to ask whether you need to write it, and, alternately, why anyone should be bothered to read it. On a personal note, I find these sort of contentless comments especially irritating when the poster is expressing indifference; the sort of twit who goes out of his way to say “::yawn::” in a comment is the sort I want first up against the wall when the revolution comes.
2. Is what I have to say actually on topic? What is the subject of the original post? That’s also the subject of the comment thread, as is, to some extent, the manner in which the writer approached the subject. If you’re dropping in a comment that’s not about these things, then you’re likely working to make the comment thread suck. Likewise, if as a commenter you’re responding to a comment from someone else that’s not on topic to the original post, you’re also helping to make the comment thread suck. On a busy blog or site, there will be many opportunities to talk about many different subjects. You don’t have to talk about them in the wrong place.
3. Does what I write actually stay on topic? As a corollary to point two, if you make a perfunctory wave at the subject and then immediately use it as a jumping-off point for your own particular set of hobby horses, then you’re also making the thread suck. This is a prime derailing maneuver, which I like to dub “The Libertarian Dismount,” given the frequency with which members of that political tribe employ it — e.g., “It’s a shame that so many people are opposed to same-sex marriage, but this is just why government has no place legislating relationships between people, and why in a perfect society government steps away and blah blah blahdee blah blah.” If you can’t write a comment that isn’t ultimately a segue into topics you feel are important, ask yourself why everything has to be about you.