I very often see comments about a question posted as answers, e.g., "format it this way" or "why don't you post it over here". I think this practice clogs up the answers and the comment feature is for these sorts of remarks. What's the general consensus on etiquette?
In addition to the basic "Answers should always provide an answer" rule, I also propose
Joke answers should always be comments
Even if the question is a terrible plz-send-teh-codez question, posting a joke as an answer is blatant rep-whoring, since you will get voted up for being funny rather than for being correct or insightful, which is what your reputation is supposed to describe.
Answers should and will be indexed by google - its what will help future generations find the right answer (via google etc). Comments - they are meta - and essentially messages to the person who wrote the question/answer (which others can view if they want to). Not even sure if the bots will crawl them.
Remarks about the question or answer should be a comment. Answers to the question asked should be answers.
Answers should add to the community knowledge on the subject. I try to keep the "it's like a Wiki" concept in mind. While I agree with you regarding answers that should have been comments, I've also seen the opposite: comments that provided useful information and should have been posted as answers or edited into an existing answer. I tend to gloss over comments most of the time so I hope that I'm not missing anything important by not reading them.
When possible comments should not be given as answers. That being said, there are people without enough rep to comment yet (like yours truly for instance :p)