As a reader, and I do read a lot, I try review every single thing that I read. Unless, it was so god awfully bad that I have nothing good to say about it. At that point, I just move along. Stories that I think are absolutely terrible or disturbing, may be exactly what someone else is looking for so I try not to leave a bad review.
Also as a reader, if I'm looking for story sometimes I'm guilty of checking the reviews to see how it was generally received, I'll probably still give it a try but reviews are kind of a way to let the new reader know what they're getting into. I believe it's our duty as a writer or reader to support the authors who spend hours of their time working on stories that they let us read for free with kind words, opinions, constructive criticism. I mean it's the only pay authors get with fanfiction and a lot of authors are serious about their pieces. Sometimes it's just a bit more than a hobby.
In my case I have a close knit group that will read just about anything I post. But they won't review it, they prefer to send me an email, or those who know me more personally a text. I don't know if it's because I write a specific genre that can be a little squicky and they don't want people to know that they read that type of thing, which I understand, or if they truly didn't like it they don't want to post a review to hurt my feelings or mislead another reader. It's quite frustrating when three people email you to tell you how great your story is, but the hit counter is at like 300. I wonder if 297 people didn't like it or just didn't want to leave a review. I accept email, text, instant message reviews because as an author I want to know what people thought of my work.