user9943: There seems to be a trend to answer questions under the assumption that comments >600 characters deserve to be posted as an answer. Often such 'comment-answers' are grasped as an opportunity for a lengthy discussion about the shortcomings of the question, the rules of Biology.SE, tips for good questions, uncertainties in the presuppositions and what not. 
Without criticizing the actual content, I nonetheless think that fluffy comments exceeding the character limit should simply be shortened, split up in multiple comments, or taken to the chat room. Answers should be reserved for answers, not for discussions on the question.
Is there an established unspoken consensus about the use of answers for long comments? And what do folks think about the (ab)use of answers for lengthy comments?
Some recent, random picked examples (nothing personal, please avoid question-specific discussions):
- Critical discussion: Gene expression and systems biology allows us to understand the control of complex traits
- How significant is the genetic component of homosexual behaviour?
- Why don't mosquitoes evolve towards muting themselves? 

user1590: Answers can be flagged for this very reason:

This was posted as an answer, but it does not attempt to answer the
  question. It should possibly be an edit, a comment, another question,
  or deleted altogether.

So the stack exchange predisposition is to flag such answers - I believe the mods can convert them to comments, but I might be wrong there. Therefore, regardless of any kind of consensus within our own biology SE population, we should not be using answers to post long comments.
I think answers should be used only to give complete, or near-complete, answers to questions. I say near-complete because sometimes it is not possible to answer all parts of a question, or the user may have good answers to some parts of it, this relates to the separate issue of having too many questions within a question.
If someone wishes to post a comment that is too long they should:

firstly attempt to make it more concise before posting the comment (remembering that, once posted, a comment can only be edited for 5 minutes). Practising the use of concise writing is important and makes the site more user friendly (people don't want to read essays unnecessarily). 
if reduction is not possible, and the comment is not grossly over the character limit, it should be split in to two comments
if reduction is not possible, and the comment is grossly over the word limit, the user should post a shorter comment, almost like an abstract of their thoughts, and invite others to a chatroom


9943: <p>There seems to be a trend to answer questions under the assumption that comments >600 characters deserve to be posted as an answer. Often such 'comment-answers' are grasped as an opportunity for a lengthy discussion about the shortcomings of the question, the rules of Biology.SE, tips for good questions, uncertainties in the presuppositions and what not. </p>

<p>Without criticizing the actual content, I nonetheless think that fluffy comments exceeding the character limit should simply be shortened, split up in multiple comments, or taken to the chat room. Answers should be reserved for answers, not for discussions on the question.</p>

<p>Is there an established unspoken consensus about the use of answers for long comments? And what do folks think about the (ab)use of answers for lengthy comments?</p>

<p>Some recent, <em>random</em> picked examples (<em>nothing personal</em>, please avoid question-specific discussions):</p>

<p><strong>-</strong> <a href="https://biology.stackexchange.com/questions/35810/critical-discussion-gene-expression-and-systems-biology-allows-us-to-understand">Critical discussion: Gene expression and systems biology allows us to understand the control of complex traits</a></p>

<p><strong>-</strong> <a href="https://biology.stackexchange.com/questions/35799/how-significant-is-the-genetic-component-of-homosexual-behaviour/35801#35801">How significant is the genetic component of homosexual behaviour?</a></p>

<p><strong>-</strong> <a href="https://biology.stackexchange.com/questions/35278/why-dont-mosquitoes-evolve-towards-muting-themselves/35279#35279">Why don&#39;t mosquitoes evolve towards muting themselves?</a>  </p>
 1590: <p>Answers can be flagged for this very reason:</p>

<blockquote>
  <p>This was posted as an answer, but it does not attempt to answer the
  question. It should possibly be an edit, a <strong>comment</strong>, another question,
  or deleted altogether.</p>
</blockquote>

<p>So the stack exchange predisposition is to flag such answers - I believe the mods can convert them to comments, but I might be wrong there. Therefore, regardless of any kind of consensus within our own biology SE population, <strong>we should not be using answers to post long comments.</strong></p>

<p>I think answers should be used only to give complete, <em>or near-complete</em>, answers to questions. I say <em>near-complete</em> because sometimes it is not possible to answer all parts of a question, or the user may have good answers to some parts of it, this relates to the separate issue of having too many questions within a question.</p>

<p>If someone wishes to post a comment that is too long they should:</p>

<ul>
<li>firstly attempt to make it more concise <em>before</em> posting the comment (remembering that, once posted, a comment can only be edited for 5 minutes). Practising the use of concise writing is important and makes the site more user friendly (people don't want to read essays unnecessarily). </li>
<li>if reduction is not possible, and the comment is not grossly over the character limit, it should be split in to two comments</li>
<li>if reduction is not possible, and the comment is grossly over the word limit, the user should post a shorter comment, almost like an abstract of their thoughts, and invite others to a chatroom</li>
</ul>
