The quality of a post is complete subjective, so the more Merit Sources and forum members in general that award merit, the more diverse criteria there will be in that definition (which is an advantage to all). There is not standard de-facto.
I personally favour more those posts that debate and express a well-versed opinion to some degree of extent, but many other sorts may qualify such as informative, witty, controversial, that expresses my opinion better than I would do, that makes me think that I may be wrong, and so on.
I personally tend not to reward mere opinions, but rather focus on the "information content" of a post.
I.e., if someone posts 100 lines of "I like this or that, it's so great", it's highly unlikely to result in merit.
If, OTOH, he only posts 2 lines about
why he likes it, there's a good chance for him to receive merit from me.