Yes you have said it all because one may not make post at all because of the fear of not making quality post so the quality of a particular post is not just something that can be determined by one user because I don't think if there is criteria in ascertain the quality of a post.
Yes, there is no criteria for quality post, because every user knows how to judge his posting quality, but well informative and educative posts are always referring to as quality post. If your post is educative, and informative, you will earn merits.
If your post help others in the forum, you will earn merits; and if your post answers or solve someone's problem related to Bitcoin and cryptocurrency, you will earn merits from the post.
Therefore, if you know these criterial, then any post you made that consist of all these information, you might be merited because it is a quality post.