Okay, if I see a quilt I find truly stunning, I am always commenting. There are some that just aren't my taste (but that's a minority, and I'm sure they will be appreciated by someone else as taste is such a individual thing). I'm too honest to just pretend I'd like them.
However, there is the vast majority of quilts that are nicely done and generally pleasant and would merit a comment. I'd rather write something a bit more personal than "nice work" or such, and that does take more time than I usually have. Partly, I guess it's also my personality - I am in general not overly generous with compliments, but I want to better and will try to post more feedback!