Nine simple rules for writing great blog comments.
Rule #6 -- Make One Point per Comment
People have short attention spans, and in my experience attention spans are shorter on the Web and even shorter when people are skimming comments. A comment should be just that—a comment—not a manifesto. If you have something so complex and important to say that you can't do it in a few short paragraphs, start your own blog. If you have two separate things to say about the video, photo, or blog post, it's usually better to break it up into two separate comments. Remember, people are often skimming.
Rule #7 -- Keep it Short
This is really an extension of Rule 6, make one point, but since it's possible to go on and on about one point, I thought I'd also remind you to keep your comments short. Again, it's a comment, not your own blog post.
Rule #8 -- Link Carefully
If you're posting a comment with the hope of driving traffic to your own site, think carefully before you include a link in your comment. Of course you should include your link if the comment box has a place for it, but leaving a link in the body of your comment is a risky thing. Many people think it's great marketing, but a minority of people think it's obnoxious and pushy.* If you decide to do it, make sure you've written a thoughtful comment that truly contributes to the conversation on the owner's site, not a useless comment that's just a transparent excuse to leave your link. It's also considered more acceptable if your link points to something you wrote that's relevant to the conversation, not just a link to your general landing page.
Rule #9 -- Proofread
I know it's hard; those boxes in which you write comments can be tiny, and they usually don't include a spellchecker. But proofreading is important because if you have a lot of typos or misspellings, it undermines your authority. Any troll who disagrees with you can just say, "What do you know about chocolate storage, you can't even spell 'their.'" If you have trouble proofreading on the Web, write your comment in a word processor where you can see the whole thing and run it through spellcheck, and then paste it into the comment box.
So that's it, the nine simple rules for writing a blog comment. Did I miss anything? Leave a comment!