I think the "competitor's product" thing has definitely been the guiding principle here. Over the years and in various threads on this board, a LOT of people have said some variation of "I never review books because I don't want to run afoul of Amazon's TOS," or "I review books but not in my genre because I don't want to run afoul of Amazon's TOS." So, like the "code" in Pirate of the Caribbean, I guess Amazon's TOS were considered more like guidelines.Bards and Sages (Julie) said:That has NEVER been the case. And I've been reviewing books on Amazon for over a decade (under my real name no less! I have not been hiding!). Amazon just finally put it in clear print I think a year or so ago, but the entire "OMG authors are not allowed to review books" thing has always been an urban legend.
The only real "gray area" is "A seller posts negative reviews about a competitor's product." Unless you are leaving one star reviews on the books of other authors in your genre, this is never going to be triggered anyway. Most authors don't leave negative reviews on other books anyway for a variety of reasons.
And I too have reviewed many, many books on Amazon-- regardless of genre. If a book is really good, I'll say so, and if it's terrible, I'll say that too. As a reader, I have that right.
Also btw, I think the OP's idea of having a "spoiler" tag on reviews is a good one. Imdb has that when people discuss movies or TV show episodes, so it would be helpful here, too.