These two seem like pretty uncontroversial uses for a comment:
- a suggestion as to how to improve the question (include a query plan, table definition, whatever);
- some of the general troubleshooting questions I'd ask in the same scenario.
You can avoid getting involved in a comment discussion by explicitly asking the post author to update their post based on your feedback, rather than leaving you a comment in return. Especially for new users, this can be very helpful in familiarizing themselves with the way Q & A format works best.
If you have nothing more to add, simply say so: if you're going to leave a comment, make it the best comment you can! For extra credit, regularly review comments you have left (via your user page - activity/all actions/comments) to see if they can be removed. If a discussion does break out, consider moving it to chat, or custom flagging for a moderator to do it for you (or take whatever other action is appropriate).
This is the more interesting one:
- a guess at a possible answer (especially ones that depend on info not provided as yet);
I understand why people do this, and in some respects it does fit in the original model:
It may help the question author (forum) but isn't terribly useful in the longer term (blog/wiki); worse, it is most often counter-productive. My view on this is that unclear questions ought to be placed on hold until they are clarified, and this should be done as soon as possible.
Guessing at an answer and leaving it in a comment has two main possible outcomes:
- The guess is wrong. Noise has been added that someone has to clean up. No one can down vote the wrong answer or edit it to make it better. Sad!
- The guess is right. The answer is now not in an answer, and the question author has learnt nothing about Q & A format (and will likely ask more unclear questions in future). Again, someone else will have to convert the comment information into a useful final state.
If you have a useful (maybe partial) answer you'd like to contribute to the community for improvement, leave it in an answer marked Community Wiki. This is quite likely to happen to answers in comments eventually anyway (if they're not simply deleted). I see this quite often: a good answer left in a dozen different comments by three or four different people. Often all that is needed to make a really good answer out of them is to add a quick code example or a documentation link.
People are already community-building good wiki answers, they're just doing it in the wrong box (comments), with a terrible UX.
As a general rule, try to ensure that whatever you contribute to the site improves it in some way (how ever small), and definitely avoid making it worse (including by making work for others).
If a comment asking a question (and leaving a suggestion of an answer) gets a response that makes it an answer, I will enter it as such. The suggestion can be a part of the clarification process; by including what could be an answer, one can prompt the poster to clarifying what they're actually looking for.
- If you're inviting them to improve their post, leave a comment.
- If you're providing an answer (complete or not), write an answer.
- There is no reason you cannot do both.
- Comments and answers (unless accepted) can be self-deleted at any time.
- Use your best judgement.
- If in doubt, ask in The Heap, or here on meta. Provide a specific example if you want specific feedback in a borderline case.