One-liners 99,99% of the time do not meet that criteria, and very few people can get away with posting something relevant in just one line. Nevertheless, piling bullshit word upon word in order to extend the post’s length a couple of lines more does not differ too much from a one-liner, and can be counter-productive.
Large posts are not that frequent, and should be justified in any case by its content. Some take posting as a personal log of all the flow of disordered ideas that come into mind, ending-up with massive walls of text that may seem entertaining at first, but that are later never read in full. Others are technical and require a thorough explanation.
What I do find is that long text from Newbies are immediately suspicious of coming not from the author’s mind, but from some other non-referenced source. That may lead to people giving the post a pass in fear of meriting a plagiarized post. Even if there is a reference to the source, I personally look for a meditated thought along with a summary, and not a pasted text with a source.
The format as you say should be simple, and normally a few fancy things may be thrown in if the text is long, in order to break the monotony. In any case, going further than using bold, cursive, or underlined text is not necessary (perhaps the odd surgical colour may be used). Capital letters and changing the default font size normally acts as a deterrent.