Length is just the same. Beyond a certain point, it becomes inappropriate for a discussion board, and more suited to other media.
Having some critique of presentation is not horrible.
Having
only critique of presentation tends to come across as over-critical, dismissive, threadcrapping,or the like. In this particular case, your point about "that's better for a blog post" reads quite a bit like, "that's a great house rule", which we also discourage as a response in the rules forums.
Is it best to have a huge wall of text? Maybe not in many cases. But it isn't a sin, either. While blogs are great for long pieces, blogs also require the reader to be looking directly at you, while a message board gets far more "passing through" traffic. If you don't have a well-read blog to begin with, a message board isn't a bad place to put such a thing.
Ultimately, if you don't care enough to read at least some of what he's got to say, you probably should just leave it be. Hobo, just below your post, is a good example. He also had problems with the presentation, but took the time to at least read and talk a little about the topic of the thread.
If the presentational aspects are so bad that they stop the reader even being able to comment on the content, then isn't that, in itself, worth commenting upon, and helpful for the writer?
Giving constructive criticism is an art, and requires a bit more finesse than just speaking your mind. Perhaps ironically, presentation of criticism may well be more important than presentation of the original post.
For example, public criticism tends to engage the ego a lot more strongly than doing it privately. So, next time, you might consider going through all the reading anyway, making a short on-topic post, and then follow that up with a PM: "I think you made some good points in that post, but next time, you might consider...."