This is a pretty subjective topic, but it is important. Reviews are so important to an author that they can literally stand in the way of a potential bestseller/world altering book ever being discovered and being a success (especially in the early stages.)
The way most people leave a review is usually based on an internal metric that readers, authors, reviewers don’t really understand. It’s either very feelings based else it comes off a very harsh rubric (did it have any stray typos, maybe a minor format error somewhere or I didn’t quite like a minor plot thing? …sounds like a 2 or 3 star rating to me! [sarc.]) I’ve looked at some reviewers who have NEVER given a 5 star review and say “I’ll leave one when I find the perfect book.” That’s garbage. Those people shouldn’t leave reviews, honestly. Their metric is wrong and those 4s should be 5s and so on. The review system is not meant to punish authors for minor rough spots in their story—it is meant to encourage creativity and buy-in from fellow readers.
Just recently I answered a random call for help from a nice gal from Calcutta whose international driving license had lapsed on a technicality. She’d been stateside for a few years but only been driving for a few months. Because of MN’s odd law she was forced to get a new license and take the driving test. We chatted about a few things (I’d only met her once before, but I am all for paying it forward) including the driving exam. In that exam the tester starts you with a perfect score and every time you don’t score a perfect on some task you lose points until you fail and they end the test or time elapses and you get a score. That may work for things like driving tests, college term papers, and some other grading systems, but not an amazon/B&N/booksellers review. We do not look for ways to fail an author—we want others to succeed!
I strive for polite reviews which help other writers sell books rather than choosing to leave critiques over form and craft. The rate/review sections of reseller pages are not the proper place to talk about things like author’s use of grammar, etc. Reviews should talk mostly of what you liked or struggled with in the context of the story. A person has to have a fundamental hate of an author to drop a 1 or 2 star review of a book (which you can see on the 1 star reviews atheists have left on my books after discovering my day job is youth work through a religious organization). People ought to remember that an amazon/B&N page, etc. is essentially a storefront. I wouldn’t approve of painting graffiti on someone’s brick and mortar store and I don’t think it appropriate for online one either. I look for every reason to leave 5 stars rather than look for reasons to chip away at a high review for a failure to achieve perfection (because so few books realistically hit universal perfection but that doesn’t mean they aren’t still perfect in their own way.)
For me, 5 stars are deserved if there is something a person would enjoy about a book (and I understand that’s subjective)… I realize that reviews and ratings are a sales tool. Even if I didn’t particularly care for content, if it’s well done and would please a target market, it deserves 5 stars. You may disagree with my opinion, but remember that review boards are not academic institutions—they’re forums to rate enjoyability. Seriously…50 Shades of Gray was awful, as were many other highly acclaimed and best-selling stories. We consume things we enjoy, not things that are perfect in craft. There’s a reason millions of people ingest fast-food garbage food instead of scientifically crafted protein cubes and nutrition wafers. We are consumers and we go after what we like and enjoy rather than what is perfect.
Really the only reason I can think to leave 1 or 2 star reviews is if the product was grossly misrepresented (maybe a book supposedly about what is wrong with our politics and capitol hill and billed as political expose which begins with great observations and facts over three chapters before veering off to talk about how the reason for all problems is an infiltration by lizard people in disguise. That book is totally fine as long as the reader is not tricked and the content description was honest).
I’d back up my opinion here by leaving a link to all my book reviews. If you want an author to keep working at his or her craft and keep shooting for the stars, then leave them 5 stars. Helping writers succeed assures you that they will keep striving for higher levels of craft (and maybe someday hit that mark of perfection.)