Of late, I've observed a noticeable drop off in reviews of my books.
Now, there could be any number of good reasons for that:
1) My reach is limited so I'm not getting new readers
2) My books don't appeal (hopefully earlier reviews will refute this?)
3) Amazon has scared off potential reviewers.
4) Most readers don't review
5) Readers aren't buying from Amazon any more
6) Amazon's new rules prohibit reviews if the reviewer hasn't bought $50-worth of product in their Amazon account, ever
It's quite possible that the third reason has some credibility. I'd recommend that if you're interested in Amazon reviews, you read Anne R. Allen's latest blog about the subject. Apart from analyzing the new Amazon review rules, there's a healthy injection of humour in there too.
And bear in mind, that reviews don't affect your book's ranking - sales do that.
Note: So far, the minimum spend relates to Amazon.Com - it would, since it quotes dollars, I guess. But watch this space...