Some thoughts on letting an author know what you think of their writing.
1/ We love positive feedback, that's the easy stuff out of the way.
2/ I personally like, "you made the following mistake in...". This is pure gold. You have paid me the compliment of taking the time to tell me how to make my creation better. I may or may not agree with you, but you have taken the time to show you appreciate my efforts AND you were paying attention.
3/ Your story is no good/full of grammatical errors/can't follow the storyline or any of the other unspecified problems. Sorry, but this is pointless. You've just told me you don't like it but not what is wrong.
As any author on this site will tell you, we write for our own pleasure, and if you appreciate our efforts that makes the whole thing so much richer. Also without a readership we cannot do what we do. Therefore dear readers - I thank you immensely.