Author of the question could optionally leave a highlighted / topped comment to accepted answer explaining why did he accepted this one, so the others can quickly see if he got it right or if something is missing. Would it be good idea?
I don't think there needs to be anything special about an "acceptance comment" from the question's OP.An acceptance already presumes that the answer is fully sufficient, and comments can be used to note further. Ideally, you wouldn't accept an answer that was deficient in any significant way.
But acceptance does not imply that the answer is 100% applicable for every possible related issue any reader might be having. The reader is implied to be responsible for assuring the applicability of any answers, guided by acceptance, votes, and comments.
Ultimately, also; I see no reason to give that one comment any more weight than any other comment that might be left.
Please, let's not make it more complicated. OPs already have a hard time realizing they can comment and end up posting extra answers.
What's the point of this? Note that not all questions are of the halpthisdoesntworkplzsendtehcodez type.. More so on other SE sites, which are not conducive to these types of questions at all.
The acceptance check tells you:
Basically, the question was answered. I went back to some of my older questions just for the heck of it, and I realized that I could not cook up a valid "acceptance comment" for any of them. Seriously. It may be just me, but I think skimming through the question and/or answer tells you exactly why the answer was accepted. Usually you don't even need to do that.
It really doesn't help people from the future much if there is an acceptance comment. If this were implemented we'd get 100% "Thanks! It worked!" and 0% useful stuff. And "It worked!" is pretty implicit if there's a green tick.
And then we'd need a 'flag as not an acceptance comment->convert to normal comment'. Which could easily become pretty heavily used.
Also, we already have this: