First, if somebody ever posts a request for a critique looking for a "Gee, you're a photographic god," he needs his head examined. You should always have a purpose when posting something for a critique other than looking for acclaim. When I ask for a critique, I'm usually specific about what I'm looking for so the people critiquing the work can help out. Some of my common themes are:
1. I like/dislike this. What do you think? In this case, I will critique my own work, state why I like/dislike it, and ask for other opinions and reasons. It helps me to see the work in a different light.
2. This didn't quite come out like I wanted. I was trying for XXX but got YYY. What could I have done differently?
3. I took picture X thinking that it would be nice but also took Y because just to see what it would look like. X didn't work but Y did. Which do you prefer? Why? How could the other have been better?
Obviously, these are pretty generic themes but they provide guidance to the person critiquing my photograph. They know what I was trying for, what I think, and can provide a thoughtful analysis of what they think. If they choose to offer something different or in addition to what I'm askinf for, great, that's a bonus.
And, most importantly, DON'T TAKE CRITIQUES PERSONALLY. If somebody gets personal, they clearly shouldn't be critiquing. Contsructive criticism is what we look for.