does your comment pass the test?
Before you post that carefully-crafted comment on somebody else's blog, ask yourself:

is my comment on topic?
An on-topic comment isn't just one that refers to the general subject of the original post. An on-topic comment specifically addresses the questions and points of the original post. If the original poster is asking why oranges are round, they don't want to hear about how you think bananas are way tastier than oranges.

is my comment as short and concise as possible?
If your response is longer than the original post, put on your editor's hat and trim that sucker down. Just can't keep the length down? Get your own blog—seriously. They're free and fun, and you'll be able to post your lengthy essay there without bothering anybody else. Leave a note (with a link) in the comments of the original blog to let them know you've responded at length elsewhere. Most blogs have a "trackback" feature for exactly this purpose.

is my comment friendly and polite in tone and language?
Even if you really disagree with what somebody else has written, you do everyone a favor when you keep things as polite and friendly as possible. Imagine that you're sitting across the table from the other person having a face-to-face discussion, and don't say anything in your comment that you wouldn't say to them in person. (And remember: people can tell when you're really being snarky and sarcastic, even if you use nice polite words to do it. So keep the tone friendly, too.)

does my comment add something useful to the conversation?
Are other people going to read your comment and think "Hey, that's an interesting take on the topic"? Or are they going to think "Wow, that guy really likes to hear himself talk"? Remember: if somebody else has already made the point you want to make, there's no need for you to chime in with a wordy "me too!"

would I want somebody to leave this comment on my blog?
It's the ultimate test: if somebody left this comment on your blog, would you appreciate it, or would you want to reach through the internet and slap them? Imagine somebody else leaving the exact same comment in response to something you wrote. If it would come across as too long, too snarky, or too mean... well, delete it and try again.

so, does your comment pass the test?
If you can answer positively to each of the above questions, chances are you have a good comment.

...post away!