If I liked something in particular I will always point that out. I will also always tell someone when I loved their story.
Very rarely (unless you have asked me to beta) will I tell you if there is something I don't like and only if I think you are a person who can take it.
Then again the last time I did it I ended up feeling ungrateful and terrible about even bringing it up. I still feel bad for telling that person that I didn't like it as much as their other stories.
I still loved the story but I'm not sure I conveyed that well enough.
So I pretty much keep my mouth shut now unless I'm asked to beta and even then I tell people that I'm going to tell them exactly what I like and dislike before taking it on.