Here are some of my thoughts on the subject for whatever they are worth.
I think most of us who provide help in these forums do so because we truly enjoy sharing knowledge and information particularly with those who are sincere about learning. And we do it without compensation.
We can't possibly attempt to answer all questions because none of us are omniscient but primarily because we all have limited time. Therefore, we have to be highly selective in addressing specific issues.
Here are some of the things I look for when making a decision about responding to a question:
a) Is it an interesting problem? Many problems have been asked and answered so many times that I feel better letting someone else with a fresh perspective handle them. Occasionally, a problem gets at the heart of some fundamental issue or concept and poses an interesting challenge. That kind of problem will get serious attention.
b) Did the student even read the problem? Oftentimes, a problem is copied straight out of textbook and, if any words are added at all, they are "tell me the answer" or "show me how to do it!" Occasionally, a link is provided to a document (Word/PDF/GIF, etc.) showing a problem being posed but without the slightest input from the student. All of these will get passed by faster than you can say "empty set!"
c) Does the student have a clue? If it's not worth the time to write the problem out and tell me your thoughts about it then it's not worth my time either.
d) Is your emergency MY emergency? Definitely not! If you have an exam in two hours and need an answer NOW then it's too late. I'd rather spend my time helping students who plan ahead and give serious thought to the problems they are working.
e) Is the student coherent? Sometimes a problem is stated in such a mangled way that it is simply incomprehensible. There are two primary reasons for this. One is that the student is utterly confused and unable to express him or herself and is likely missing some basic underlying concepts or information. I'll ask for more information in most cases and go from there (I've resisted the temptation to suggest, in some cases, that the student shouldn't be in that course in the first place and should go back and acquire the prerequisite knowledge!)
The second reason for incomprehensibility is language based. The forum is in English and, for many of the participants, English is not their native language. I applaud them for their attempt to communicate and for their interest in physics and math. I will make an extra effort to understand what they are trying to say.
f) Student with attitude? If you're demanding that someone drop whatever they are doing to provide you with answers RIGHT NOW then you're going to be very disappointed.
Finally, I'll summarize with this advice: If you would like to get help from people who are volunteering their own time and talent then you should state your questions clearly, demonstrate that you have seriously thought about the problem and made a serious effort to solve it and be prepared to follow up on hints and suggestions.
I think most of these ideas are expressed in one way or another in Tom's policy statement referenced above. From an "advisor's" perspective, they make good sense.
P.S. Oh, by the way, it wouldn't hurt for students to thank HW helpers occasionally for their effort! :)