It's a given less than flattering reviews are going to happen on even the best selling and outstanding books - a fine example being the Twilight series, which had more than its share of negativity, yet continues to have strong sales and even made into successful films.
Among the four and five-star reviews I'd had on The Cruiserweight, there was a two-star rating, but the actual review of my book was done in such a mature, honest and insightful manner that I respected the reader for taking the time to express his opinion. These type of reviews are the exact kind which helps potential buyers make an informed decision - this particular review included both what the reader liked and disliked, was specific, neither rambled on too long nor just had one short sentence, and most important, was sincere.
On the other hand, there's been so-called reviews which bordered more on cruelty than a proper critique. For example, I had a troll some time ago who said my book was 'rape porn' and printed personal attacks about me, all while using vile language in their "review." It was clearly obvious they'd never purchased the book, let alone read it. Of course, after the latter was reported by several readers as violation of Amazon's review policy, the offensive content was promptly removed from my book's page.
These types of reviews shouldn't be taken seriously. Ninety-nine percent of the time, they are written by amateur "critics," perhaps those who have underlying jealousy issues. In any area of the arts, this is a common occurrence. What really makes me shake my head in bemusement are the "reviewers" who spend more time picking apart every aspect of grammar and less on critiquing the book's actual story, plot, characters, etc., all while not knowing the difference between drivel and a brilliant story. In these cases, we all have to consider the sources and rise above the nonsense.
There were authors accused of having 'fake reviews' on their own books in order to get higher rankings. While this could be true in rare cases, for the most part, I've read several Amazon reviews and found most to be thorough and genuine.
A publisher pointed out that bad reviews are a sign of success. No one bothers denigrating a nobody. A book with all five-star reviews tend to make some people a little suspicious in contrast to mixed reviews. Criticism is part of our creative process, but we also have to remember there are still those with the sheep mentality who want to read what is "safe and acceptable" and any book outside of those lines are subjected to smack downs, so to speak.
The best thing we as authors can do is push forward. Not everyone is going to love us and our work, so it's best to shrug off the non-flattering reviews instead of becoming emotional about them. As writers, we need to have strong spines and thick skin while distinguishing the lines between criticism and cruelty.