Quality posting is what people do when they are genuinely engaging with a forum and not just posting to fill a quota. I don't consider a one line post that generally adds nothing to the overall discussion as a quality post. It's generally not about the post length, you absolutely can sum up your thoughts in a couple lines, especially if you're making a reply to a query. However, a lot of people tend to come along and just throw in a two line post for the sake of it and it's clear that they've read none of the other replies in the thread.
As you said, something which adds value. I don't believe length has anything to do with it. If anything, I prefer shorter posts that get straight to the point. Nobody wants to read a five minute paragraph when the answer is either yes or no.
Why on Earth would anyone post a thread that has a yes or no answer though? We've actually deleted threads like that in the past when we had a much younger userbase. Instead of just deleting it though, giving them guidance on how to remake their thread and turn it into something that people can have a real discussion on. That's how younger members learn to make interesting threads. Allowing multiple threads with no discussion value really brings down the quality of your forum and leads to very little activity.
Then there's the classic one line "I agree with this" type posts. Those posts are spam. If you're going to go into a thread just to post "I agree" with the thread title, don't bother posting at all. Seriously, lol, I feel quite strongly about that because people who post like that are generally just trying to raise their post count and are not interested in contributing to the community.
Having said all that, you do have the fake intellectual who writes monstrous posts in an effort to confuse their "opponent" after they discover that their original stance on a subject was wrong. They pull strawmen out of the hat and write essay length posts in order to twist the argument around. Those posters are incredibly annoying too, and detrimental to a site as they alienate other members.