I don't mean that their posts are too long, because sometimes the problem is that the post is too short. If you are comparing, say unicorns & rainbows, don't explain that they're different, expound on unicorns, and then forget to talk about rainbows. It's like flirting all night and then forgetting to get my number. Put paragraphs in to separate different ideas, even though your ideas might only be one or two sentences long.
Other times, the posts are too long. I suffer from this problem a lot. It's taken me far longer than planned to dash out my thoughts on this. Some posts meander in several directions before they finally die a slow death. Other times, it just needs a good editing to cut out the extraneous verbiage.