A few hours ago, I commented on a question with a simple example for OP. It was a partial answer, with no evidence, no research, et cetera. I was told that I should refrain from answering in the comments.
More recently, an OP asked whether certain studies had taken place. I quoted a lengthy but terse summary from a scholarly paper, listed the papers that that summary referenced, and then linked to the aforementioned paper. I was told I should have added it as a comment.
On Stackoverflow, I provide "answers" in comments all the time. Sometimes the question has been asked a million times already, but more often I simply don't have the time to do the research to provide a solid, filled-out answer (like in the first case above). My comment can provide a launch pad for OP to go help himself, if you will. Most of the time, I have no intention of ever coming back to the question to give a full answer - I'm too busy.
Also on stackoverflow, "link only" answers are discouraged. Instead, users are supposed to link to the source page and provide relevant quotes from the source in case the link dies. This type of answer is very common, even without any extra input, and I've never seen it frowned upon. In my particular case, it was pretty much what OP asked for.
What is the dividing line between a comment and an answer? And if there are sources with a good answer outside of this site, is it OK to link to them and provide the relevant snippets, or should you always provide your own thoughts as well?
