I have checked a random sample of questions in the main page, and it seems that almost every one of them has some form of answer in the comments, apart from completely unanswered/untouched ones.
This meta post mentions an old tea discussion (EDIT: added archive.org link, thanks to Martin Sleziak for suggesting it), and the author concludes that "[s]ome people sometimes prefer to answer in comments, and there is not much that can or even should be done against it". I'd like to revisit this, however, since the practice has reached endemic levels.
Just to summarize various drawbacks of answers in comments:
- comments cannot be accepted. Questions answered only in a comment get periodically bumped up to the main page by bots, producing noise.
- comments cannot be edited and improved/corrected.
- the previous point applies also to automated edits to fix broken links.
- comments cannot be searched using the search box.
- comments cannot be downvoted if they are wrong; the only way to dispute the content of a comment is starting a long discussion in a comment thread.
- comments-to-comments often produce long and messy threads, which could then be moved to chat, together with the initial answer-as-comment, and become even more difficult to locate.
- comments are ephemeral and can be deleted by mods without trace under some conditions.
- it is not clear which version of the CC license applies to comments.
There is apparently an unwritten tradition that answers that are very short, or just hints, should be written as comments (probably for fear of 'unwelcoming' downvotes?). My opinion is that this tradition needs to change, to avoid these drawbacks. Even bad and link-only answers should be answers, not comments.
What can we do, and what should we do, to address this phenomenon that is getting more and more widespread?
EDIT to clarify: I am speaking about questions that are on-topic for MO, not questions that are just about to be closed.