First, the image sharing section is not meant for critique but for sharing. Only if you ask for some specific criticism there, will you receive some. If people don't ask for it, it is usually a bad idea to be giving it. More often than not it is not appreciated.
But as Woolburr already pointed out, even when people specifically ask for constructive criticism they can become totally defensive upon receiving it. It is most annoying which is why I think twice before giving honest criticism, especially if the person is new to photography.
I hang out in the birds forum mostly and when I see newcomers I watch them a bit, see how they respond if someone (could be me even) gently gives them a pointer on how they could improve their images. Occasionally a person will respond brilliantly, will accept any and all criticism cheerfully without ever going on the defensive. As a result their progress is often amazing.
As to the 'Nice', "Very nice", etc. comments in the Image Sharing section. Well, there is a bit of a code I think. First of all, when you have been around here for a bit, you start making friends. Sometimes they will make great images, sometimes they will make mediocre images. Just like myself. But you still want to leave a comment because they are your friend. And so you say 'Nice' or 'Very nice'.
However, when the images are really wonderful and a league above 'Nice', I promise you you will see different comments.
I know that if I start a thread with images that get the friendly 'Very nice' and 'Nice' comments, those images aren't very good and it's just my friends being nice to me. My really good images get very different comments. In essence, you just have to learn to read between the lines there... 