Keep it short
Blog comments are often no longer than a short paragraph and some can be just a couple of lines. Your reader will not stick around if you are rambling, and people won’t read your comment if it looks too long. Of course, occasionally you may want or need to write a longer comment – but as a rule, keep it short.
Keep it focused
Making a blog comment is an exercise in focused writing. If you are rambling and incoherent, and if readers can’t follow your train of thought, then they won’t read your comment.
Write simply and clearly
Blog language as a whole is less formal than essay language; however, you must still use correct spelling, punctuation and grammar. Use simple, jargon-free language, and make clear statements about your ideas, reflections, opinions, commentary, whatever.
Express your opinion
People read blog posts and comments to get opinion. Don’t shy away from writing what you think, but make sure there’s some critical reflection going on – not just rant or unfocused ramblings. Comments are not the place for mucking around, so make sure you comment intelligently.
Put some of yourself into it
Blogs provide a less formal type of assessment than do essays, so you’re allowed to put a bit of your personality into your comments … just make sure that that personality isn’t too irritating. Humour is good, as is a bit of emotion or passion. But be sensible in your writing, and be aware of things that might offend or put other readers offside. If your comment is ‘off’ or inappropriate, the blogger may reject it.
Edit your comment
Edit for style, tone, grammar, spelling, and focus. Poorly structured and worded text will put readers off.
Here is some advice from the Good comment website
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.