ServerFault's FAQ gives some indications on what comments should be used for:
If your question needs clarification, you will see comments in smaller
type below your question.
That is the most common usage of comments, by far.
Above all, be honest. If you see misinformation, vote it down. Add
comments indicating what, specifically, is wrong. Provide better
answers of your own. Best of all — edit and improve the existing
questions and answers!
That's a close second, in my experience. No other clear indicators of comment usage is given in the FAQ. So it appears that comment are mostly intended to cull more information from the original poster or to critique a poor question or answer. Usually poor questions are critiqued with "this does not belong here," and poor answers with "you will destroy someone's system, because..."
Also, read the FAQ and see how many times there is an emphasis on "better answers" and "good answers." Nothing is implied that the answers will be a direct answer to your question. Just that StackExchange is interested in quality, not scope. Good answers are preferred to tightly scoped answers. This is true for all of StackExchange, insofar as I know.
Allow me to comment on specific things in your questions:
Perhaps it is different in serverfault than the rest of stackexchange?
I haven't seen evidence that other StackExchange sites have a stated use for comments that is any different from ServerFault's.
My impression was that a response should be a comment if you don't
directly answer the question. However, a more seasoned serverfault
Responses should be comments if it doesn't answer the question. Notice I removed the word "directly." If someone asks how to develop the next Facebook using their mobile phone as a server, we are likely to answer it, not with architectural designs that involve OMAP-based phones, but with... different advice.
Here's an example from a very recent question I answered titled "Confusion when configuring exim or postfix." A fellow wanted to know how to configure an email server, but instead of answering that direct question I told him that he probably doesn't need to install and configure an MTA. Rather, he should consider just using the mail functions of whatever CMS / language he's using. Others seemed to agree with me since, as of this writing, it's the highest rated answer.
However, a more seasoned serverfault user disagrees.
Looking at that interchange, the topic is pretty well summed up. In fact, I like Womble's screwdriver analogy a lot. =)