I agree with the first two paragraphs of the answer by Nij: a general comment explaining how acceptance works, particularly to relatively new posters or those who may not understand the mechanism, is acceptable.
I do not agree that "A comment requesting acceptance of a specific answer, or even to a specific question, would not be appropriate." at least not in all cases.
I agree with the part of the answer by Greendrake that says:
I do not see anything wrong with a comment seeking to find out what makes a particular answer still not acceptable despite its apparent quality.
I do not agree with the part of that answer that says:
Upvoting should be seen as the primary source of encouragement for thoughtful responses. Whether the OP finds them acceptable should be of a little concern.
If an acceptance were not designed to be of concern to an answerer, it would not carry reputation points, as the upvote of a comment does not.
When a user, particularly an apparently new user, has posted a "thank you" comment to an answer, I have sometimes responded that the best possible thanks is an upvote, and if the user thinks the answer is THE solution, acceptance. I include a brief explanation on how to upvote and accept, and usually mention waiting a day or so before accepting in case a better answer is posted. This seems acceptable to me.
I have not noticed a particular asker who never or rarely accepts an answer. But I do not se why a single polite question about such a practice would be unreasonable. Pestering o repeatedly bringing up the issue would be, I think.