Plenty of critique has been written about the art of writing a good blog. It seems, for the most part, we want blogs with substance, that bring up issues that matter, tell us why they matter, and offer an opinion based on facts surrounding the issue. However, the blog community (I will not say blogosphere. I will not say blogosphere.) is useless without a base of people who can respond and argue effectively.
In case you haven't noticed, this isn't your shitty .php forum on newgrounds.com. If you want to respond to a particular blogger's comment, you don't have to write "in response to [comment title]." You don't even have address the blogger by name. What this (and many other) site has done is instituted the reply button which allows your response to be tiered right under the comment to which you're responding.
An effective comment does three things: it addresses an argument (or arguments), challenges it with an opposing view (I've seen some comments which don't), and most importantly offers facts, quotes, logic, and/or expert opinion in order to support the aforementioned opposing view.
So, for example, a comment such as "you're wrong" or even worse "ur rong" does not add to the discussion at all. A decent comment will be at least two sentences and probably no more than 3 paragraphs. If the comment is shorter than that, chances are, you haven't said anything of worth (or you have amazing grammatical skills). If the comment is longer than that, chances are, you should be writing your own blog entry instead.
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I disagree with you saying a comment must challenge an arguement with an opposing view to be effective. Some comments build on what was said in the post and/or previous comments, rather then directly opposing them. Such a comment might go something like "I agree with your plan for A, B, C, and D. I would also use it in conjunction with plan XYZ to prevent EFG." This type of comment can provide as much useful input as an opposing view point. I would even argue that a comment which builds on previous views is more useful then an opposing view, since you spend less time arguing and more time finding solutions.
Many writers also appreciate just being told someone read and enjoyed their work once in a while. This type of comment may not directly benefit the community, but providing someone with encouragement indirectly benefits the community when that person continues trying to improve their writing and make a difference rather then getting discourage by negative comments.
Providing opposing views is important, too, but that is not the only type of comment that's effective.