To be honest, it is a subjective term but considering the mass audience, we have generalized it and framed some parameters to classify any post as high quality. I have seen many posts that have added value to my knowledge but gained almost no merits, on the other hand, there are some worthless posts (in my opinion) are appreciated by some people. According to me, a general high-quality post is something that adds relevant value to the knowledge of the mass audience irrespective minor grammatical or layout mistakes (I am not supporting mistakes, one should always keep improving but I won't ignore the value of the post just because there is some grammatical mistake).
I think your opinion has similarity with mine that sometimes merit is given to worthless posts while some good posts has not been merited, even I see also some threads that open the opportunity for member who feels deserve to receive merits with their posts to be reviewed by the OP. I think if it is getting more threads specially opened to review unmerited posts, here below the beneftis:
1. The objectivity problem will be reduced
2. The purpose of merit system can be maintained
3. Members will still be encouraged to post high quality
4. Merit transaction will be reduced also as there are lot of "free" merit giveaway with really objective review for high quality. Members can be attracted to receive merits by posting in high quality rather than just buying merits.