I spend far too much time writing comments on blogs and have been doing so for years. Recently I started to think about the kind of comments that I think are constructive and actually lead to someone learning something. I came up with a few rules – mainly to remind myself, I know I often break them – but I wonder what others think?
Stay polite. I know the other guy is a bigoted moron. But you won’t change his views by telling him so, or change anyone else’s. If you really think that little of him then why waste your time? Debate with someone else.
Use the standard rules for good writing. Short words and short sentences. Active voice. Concrete nouns and verbs.
One point at a time. It is always tempting to air your favourite quotes and thoughts about all sorts of subjects. But think about the person who is reading your comment and responding. It gets really confusing if there are several different items in the comment. If necessary write more than one comment.
Keep it short (less than 500 words). Some of us have trouble reading more than 500 words from a screen. Anyway you will probably make your point better if you edit it down. Another way of putting it is – if the reader has to scroll down to read it all then they probably won’t.
If the reader doesn’t understand, assume it is your fault not theirs. It takes two to communicate and the writer has the prime responsibility for making it work. It is difficult to write clearly and if you aren’t working hard you probably aren’t succeeding.
Don’t go over old ground. It is a waste of time and very, very boring.
Know when to stop. No one ever had a change of worldview in the middle of a blog discussion. Make your point, understand theirs, then get out and let the thoughts take root and germinate – maybe one of you will have an insight one day.
Use references to help not to impress. Sometimes a blog debate can turn into a sort of war of references – who can make the most references using the most learned sounding sources. It doesn’t help. The reader will ignore most them anyway. If you think a reference is helpful, explain why you have included it, make sure the reader can read it without paying, perhaps point them to the relevant part if it is a long one.
Some, but not all, of these rules apply to the initial post as well (like this one). For example, posts should use standard rules for good writing. But they can be longer.
In Moderation 
Those are all pretty good points. And I would say that probably apply to online forums, as well as to blogs.
I try to keep in mind that in an online debate, I am not trying to persuade my opponent; that’s often impossible. But I am trying to leave a clearly stated position to be read by those who may be following the discussion, but not actively participating.
Very good points. I think to see how not to write good comments one just has to look over at UD at Bornagain77 (thank goodness for the page down key!) and Kairosfocus (blinded by his own brilliance…but a completely ineffective communicator)
ba77 suffers from what used to be called “diarrhea of the typewriter.”