I am unsure about is why at some times people post a comment instead of an answer, though they do in a sense seem to answer the question in the comment. Are these people wrong in doing this or am I not seeing when it is more appropriate to comment than answer?

For example, this.

If I am mistaken, please explain how I can differentiate this for myself.

Commentators comment because they comment! They usually have their own reasons, but the reasons usually boil down to this:

• They're requesting clarification

Without some context, it might be hard for us to help you. Could you provide a link to the source of the data or something similar? A Google search for Kdep turns up a radio station and a DJ. - Ben "cool chemist" Norris

• They're kinda explaining the reason of upvote/downvote, which may result in post development

These sometimes don't contain the reason:

Very interesting question (+1), I look forward to reading some answers to this. – User "Anon" 4076

Or they may:

Answers involving purely angular momentum considerations cannot be correct, as they cannot explain why the 2s and 2p orbitals are degenerate in energy in the hydrogen atom. - Acid "Burning" Flask"

• They do something an answer shouldn't do

Please have a look at the web site of Artem R. Organov and read the 2013 paper in Science, where he outlines the synthesis of $\ce{NaCl3}$ from $\ce{NaCl}$ in a laser-heated diamond anvil cell in the presence of excess sodium and chlorine. He suggests that under these conditions, rather unusual $\ce{Cl3-}$ anions are formed. - Klaus "Voter chemist" Warzecha

An answer shouldn't be "link-only". When Mr. Warzecha didn't have the time to provide an answer, he directed me to a link. This is how comments work, and are supposed to work.

• They explain the policies of the site/welcome new users to the site

This is a homework question. We have a policy which states that ‎you should show your thoughts and/or efforts into solving the problem. It'll make us certain that ‎we aren't doing your homework for you. Otherwise, this question may get closed.‎ - Me "Normal me" Semi-chemist

Coffee is brown either because it contains tannins or melanoidins (multiple sources site each...) Both are large organic molecules, so if you can break them down into smaller organic molecules you should be able to decolorize them. The easiest way to do that is as @Martin describes, via oxidation. You might be able to do it with something like OTC hydrogen peroxide even (or OxyClean). I'm not sure enough of the method to write this as an answer, but it's worth a try. - Jason "ELL" Patterson

• They are moderator/editor notes about an issue a question has

This was untagged, if anyone has better tag suggestions, please go ahead and suggest them/make them. - Jonsca "Organochemist" Moderator

This is rare, and happens mostly to problematic posts. In a relatively smaller community like chemistry, it's even rarer.

• They are funny stuff that are too cute to be flagged

This usually doesn't happen here, but on meta.SE. I'll pass.