Everything else such as 'me too', 'thanks', 'asdfgh', 'hi mom!' etc are cut and dry enough to not need explanation. If you think a one line answer is useful enough to keep around, but doesn't constitute an answer just flag it as other and indicate why it should be a comment.
Moreover among the comments of this answer from @animuson he explained to me that I should have custom-flagged a too-short question I simply flagged as NAA at the time.
Therefore when I find such answers usually I custom-flag them after leaving a comment like this:
This answer is too short and doesn't meet SO quality standards. Please improve it by adding some explanations in order to make it useful for the community and not only for the OP.
However I recently gave a closer look at my flagging history and noticed that several flags I raised following those guidelines were declined with various motivations, among which:
declined - flags should only be used to make moderators aware of content that requires their intervention.
declined - Just because an answer is short doesn't mean it should be a comment.
declined - flags should not be used to indicate technical inaccuracies, or an altogether wrong answer
Therefore I'm a bit puzzled now. Did I miss something or simply there is no consensus among moderators about what "too-short answers" mean? Should I continue custom-flagging too-short answers?
Given the answers/comments I'm getting I realize probably I haven't been completely clear. My puzzlement is due to the fact that many times I have flagged answers that I deemed too short (according to the links I provided) and those flags were accepted as helpful. Therefore when I analyzed my flagging history in more detail I found that some mods had different opinions about that. Thus my question here.
So I must infer that different mods have different opinions about flagging too-short answers and no clear-cut rule/guideline is actually in action. This question is not about ways to improve an answer, it is about how to become a better user by flagging appropriately when it is needed. Clear guidelines are useful for that. If there are none, I'm ok with it and I'll take it into account when flagging.
I see two possible cases about flagging too short answers:
There are clear-cut guidelines, but I haven't been able to find/understand them well enough (in this case I'd appreciate some more pointers).
There are no clear-cut guidelines, and the discrepancy I found in the mods reactions to my flags is the obvious consequence of this.
If there is another explanation, I'll be glad to know what it is.