I always try to stay positive...if I can't say anything constructive, I usually don't say anything at all.
I will, if I see a problem (bad seam, color values, etc.), I'll make a suggestion that will be constructive on the next quilt.
For example...a flying geese block that has the tip cut off...I'll say something that I like about the quilt, then mention that I have a problem with having to remember that when stitching seams, to make sure all stitching lines cross exactly, so I don't inadvertently cut off a point. I'll also mention that if I do cut off a point, when I secure that block with quilting, I don't follow the cut off point, I quilt a point...it throws the vision off and you don't really notice the cut off point.
I try to complement & give a quick lesson in the process. You don't want to discourage a person who may/may not have the knowledge on how to do a certain block or method...you want to teach them.
I'll also make color suggestions..."Your colors are very nice, have you thought of perhaps trying this..." Then I go on to mention something about value.
There is absolutely no reason to just plain be rude and put down someone's work, just to tout your own skills...be nice and teach when you make a comment.