Q:

Restricting comments.... (and sometimes allowing longer comments)

This is a related meta-question, and so it this one. But my particular meta-question is focused on comments only (mostly comments in questions).
First, I strongly believe that comments should be disallowed, or very restricted, on owns questions. So I suggest that the web interface forbids commenting your own question (unless perhaps for the few people having a lot of reputation, at least 10K). Almost every comment by the OP on newbie questions should actually be edits to that question (because newbies tend to ask too short and very imprecise questions, and don't spend enough time and effort - and wording - on their newbie questions).
Then, for a few people very used to SO (perhaps with a score above 50K) there is sometimes an occasional need to write a comment longer than the 500 character limit which is not an answer to the question. Maybe we should introduce some "large comment" interface for them? And these large comments could have several paragraphs, embedded code, etc... exactly like answers.
Another way to do that could be the ability (for experimented contributors) to add a comment-like answer (like this) and flag it as such.
At last, the interface should explain more about comments (perhaps by putting some grey explanation in their background, or some pop-up, etc). My initial perception of comments is that they are not stable (and e.g. might be automatically removed in a few months or years), in contrast to questions and answers.

A:

As mentioned there are in fact 2 different features requested. Let me give you my opinion about both:

First, I strongly believe that comments should be disallowed, or very restricted, on owns questions. So I suggest that the web interface forbids commenting your own question (unless perhaps for the few people having a lot of reputation, at least 10K). Almost every comment by the OP on newbie questions should actually be edits to that question (because newbies tend to ask too short and very imprecise questions, and don't spend enough time and effort - and wording - on their newbie questions).

I agree insofar, that most of the time if newbies are asked for clarification or additional code details in comments, they tend to react putting this additional information into a responding comment directly.
This wide spread behavior produces unnecessary noise and is annoying.
Though I don't believe this should be really solved by having a restriction to comment on your own questions at all. So even if the OP edits their question in response, how could he ping the commenters, who asked for clarification, to make them aware of their edits?
Often enough this is simply solved by some good souls who

Leave a comment to make the OP aware that they should edit their question instead of commenting
Pick up code or info from the comment and edit it into the question

Then, for a few people very used to SO (perhaps with a score above 50K) there is sometimes an occasional need to write a comment longer than the 500 character limit which is not an answer to the question. Maybe we should introduce some "large comment" interface for them? And these large comments could have several paragraphs, embedded code, etc... exactly like answers. Another way to do that could be the ability (for experimented contributors) to add a comment-like answer and flag it as such.

No. I strongly disagree about the usefulness of such feature. If we have comment like answers or answer like comments what justifies this narrow line to draw?  

The question asked for an opinion? -> Close the question (off topic as primarily opinion based) 
The question isn't clearly answerable? -> Close the question (off topic as too broad)

Sometimes there are "questions" that would lead to very interesting discussions or evolving theories. But even if interesting (especially for higher level experts), these simply don't fit for the intended Q&A format of Stack Overflow. There's a word going:
Not all questions are a fit for the Stack Exchange format.

