Once I posted a picture and somebody commented on it in a way that made me think that the title doesn't suit the picture, so I went ahead and changed the title. And after that I saw that people's comments were more appropriate to the idea of the picture.

Sometimes  I leave feedback on images that amazed me in some way and I try to specify what particularly I like. More often I just enjoy a picture and it's hard for me to pinpoint the reason, so I just exclaim.
There are instances when I see an annoying photoshop effect or other quality that I don't like -- but I see that an author is really into using it throughout his/her collection, in that case I just ignore that author -- that's a matter of taste.

Another matter of business when an author is asking  for constructive feedback... but I haven't seen those often.