There are two reviewer types I'd like to address: formal and informal.
I've emailed several book review bloggers and received a consistent promise to be honest. Another consistent (but less so) promise is to never write anything bad. As an author, I genuinely appreciate the graceful bowing out of a reviewer who just might not like my writing style. However, as a reader, I worry about some of the reviews I've read on Amazon.
I'm reading several books right now but only one based upon Amazon reviews. The book received 4 and 5 star reviews with glowing praise about the plot, so I eagerly began the story.
Then I eagerly stopped.
Repeat after me, a reviewer should be honest but not brutal. Do not shy away from discussing the weakness of a story in an effort to be supportive. I'm not suggesting that a reviewer cover a copious list of grammatical, structural, and emotional check points for each book. But if the work has an obvious weakness, acknowledge it because the reader will hold you responsible for not being honest.
I repeat emphatically: do not mislead me. The story may have a great plot, but if the execution is not excellent, don't just omit that important factor in order to give a positive review. The book is published; the author has signed off and said, yes, this is my best. Supporting a writer is more than being a cheerleader. It's helping people to produce their very best.
If a book is in process, not published or seeking pre-publication reviews, the type of review desired is often very different. These are often performed by followers, friends, and/or interested parties. So what does an author want? That is a question best asked of the author.
In my opinion, if an author is posting a work in progress, they want to know if you're interested in the plot. Do the characters intrigue you? Would you want to know more? If there are obvious grammatical or syntax issues, ask privately if the author is aware. It sucks having to moderate comments.
I've been writing my novel for years and the most beneficial comments were regarding theme, plot, consistency, voice, dialogue, etc. Mostly things that come from the soul of an author and don't have exact, nearly unbreakable definitions. As I writer, I know I struggle with commas and clauses, run-on sentences, and fragments. Grammar and syntax editing can be purchased, if necessary. These things are only fair game if the author requests such a review or if they've published their work. Once a book is published, the gloves are off.
Some informal reviewers won't be able to read your story if you have consistent grammatical errors. For them, it is as egregious as poor plot development. So when you seek readers for a blemished piece, prepare for pimple poppers.