1.)I would give them an honest review, they wrote it and put it up, they should be able to take the fact that it may not be the best story. I would give contructive critism, though, not just come out and blatantly say "this sucks" or lie and say "its amazing".
1.)I think reviews are to tell a writer how well(or not well) they wrote their story, and also to help them improve. Ratings, I think, are just their to say either "this was good" or "this was bad".
3.) I agree with the above on this one, but sometimes its just.. Awful.
4.) I'm very OCD about grammar, and I find it hard to read stories that have improper grammar. I will make exceptions if I think the story is worth it, but if its cliche and kinda lame, then no I won't read it. Even if the writer says its terrible.