“…amateur book reviewing technologies like Goodreads have democratised the practice of literary criticism. Where criticism was once the exclusive domain of the learned professional, book review websites have enabled everyday readers to take part in the critical conversation.”The Amazon/Goodreads review system is not perfect. There is room for abuse, and there is abuse.
At one end of this continuum are sock-puppet positive reviews planted by an author's friends, family, and loyal blog readers. (The last of these comprise more of a gray area, but there is a real issue of objectivity here.)
At the other end are negative reviews planted by people who simply disagree with author's views. (I.e., the author is too "liberal", "conservative", "feminist", "sexist", etc.) There have also been cases of authors writing negative reviews of competitors' books.
Still, since readers--as opposed to professional critics--are the ultimate end users of every book, it is fundamentally a positive development that they now have the largest voice in the "market" for reviews.
Over time, honest reviews of a book are likely to balance out the disingenuous ones, be they positive or negative.