AlanKlein
Been spending a lot of time on here!
I enjoy getting a "nice shot" if not a longer critique. I admit I post my shots out of pride, but hopefully not too prideful. It's easy to get your mom to tell you you're Michelangelo. Getting similar respect from other photographers is a bit harder. I try to give that back as I know that it takes courage to post a shot and hard work too. You wouldn't post it if you didn't feel it meant something to you. So thanking someone for posting it and giving him some measure of respect is worthwhile. I think getting no response at all is worse than anything. You wonder if anyone actually looked. 
Having said that, giving good critique is very important. I'd like to know what people think. I may have missed stuff, probably often, because it's mine. If you look at someone's portraiture, and tell them the eyes would look brighter and more alive if they added catch lights, that's a positive learning experience for them. The art of critiquing is to be positive and gentle, something I find hard to do if I get carried away. Criticizing someone nicely is hard. Be nice when you do it.

Having said that, giving good critique is very important. I'd like to know what people think. I may have missed stuff, probably often, because it's mine. If you look at someone's portraiture, and tell them the eyes would look brighter and more alive if they added catch lights, that's a positive learning experience for them. The art of critiquing is to be positive and gentle, something I find hard to do if I get carried away. Criticizing someone nicely is hard. Be nice when you do it.