I recently spoke to an author who is published by one of The Big Guys. She was under pressure to get a minimum of 60 reviews for her latest book within 30 days of its release. Because it was her first book, she was understandably stressed.
In today's market, it isn't enough to be published - after all, with today's technology, it's easy for anyone to publish their own work.
The difference in whether a book will be successful depends largely on visibility. Reviews are one of the ingredients in the mix. The more reviews that appear for an item on amazon, for example, will often affect where it appears in a search and whether it's a recommended read.
Negative reviews can tank a book, especially when there aren't a sufficient number of good reviews to offset the bad. For that reason, I won't review a book I didn't like. I wouldn't want to be a participant in harming an author's career.
Many print reviewers (magazines and newspapers) have told me they also have stopped negative reviews. Their reason: they only have "x" amount of space and why waste it telling their readers what not to buy? They prefer to focus on recommending those books they particularly liked.
What was the last book you reviewed online?