What is the correct way to deal with someone who posts an answer largely echoing material put forward in a previous 'comment' (possibly because the 'commenter' wanted to help the questioner, but felt the question not quite appropriate for ELU)?
I do sometimes restate as a formal answer the answer that a commenter has provided in response to a simple question, but I generally try to observe three points of protocol in doing so:
I focus my efforts on questions that I find in the Unanswered Questions queue. Any question that already has an answer with a net positive vote score will not appear in that queue.
I wait several weeks before offering such a restatement. This ensures that I'm answering a question that isn't on the verge of being closed, but instead shows signs of lingering indefinitely in the Unanswered Questions queue, taking up space there.
I give explicit credit to the commenter who provided the dispositive answer to the question.
Truth to tell, the answers in most such cases don't redound to the credit of either the commenter or me because they are obvious and elementary. But perhaps seeing an actual answer to the question will encourage another reader not to ask the same one (assuming that such readers search for such questions and answers before posting the same question). Or perhaps someone will upvote the obvious answer, and we'll have one less piece of dead weight in the queue. Or perhaps, as is usually the case, nothing will happen—but at least we will have a formal answer to the question in place.
Interesting and challenging questions do sometimes lurk in the nether reaches of the Unanswered Questions queue. And I think it's a shame that those questions are surrounded by a mid-Pacific Gyre of dull and incoherent questions that repel any but the most dedicated question answerer from seeking them out. I don't have a solution to the problem of what to do with the junk besides to try to retire it, but I do think that the Unanswered Questions queue is well worth inspecting regularly in hopes of spotting an interesting question that somehow went unanswered (or unupvoted) on its first descent through the EL&U main page.
As Josh61 noted, comments are comments. Answers are answers. If you think your reply will answer the users question, take a few seconds to find a link to a reliable source and post it as an answer. I believe comments are for clarification and further discussion, not for answering the users question. Any time you do that, it's fair game and you are running the risk of someone hijacking your content. Yes, the answerer should reference your comment if they are directly copying it, but there are no rules saying that "if a commenter got to it first, you can't use it as an answer," and it should stay that way.
Basically, if your answer is an answer, post it as one.
I'm going to go with: Not starting a fight over nothing by making condescending comments on perfectly legitimate answers and in doing so derailing an actual conversation that was taking place.
Refer yourself to Andrew Leach's comment for a duplicate meta question which contains overwhelming support for my stance.