If you've been checking out blogs for the last year or two, you'll see more and more sites with side bar's highlighting the blogger's favorite books and CD's. While this is good from the point of view of a browser looking for product recommendations, it should be troubling for Amazon.com.
The feature that really differentiates Amazon.com from all of the other commerce sites out there is the community element. Throughout Amazon.com's history, thousands of people from across the internet have been willing to spend significant amounts of time writing interesting and thoughtful reviews of books and products.
Part of the reason people chose Amazon.com to share their thoughts was the ease in posting. In the pre-blog days of the internet, creating your own page was tedious and out of the grasp for most people. Amazon placed member reviews on actual product pages, so everyone had the chance to be an influential reviewer.
In this blog age, however, people have less of a reason to write reviews for Amazon.com. People with a distinctive voice and an inclination to write can easily have their own site where they can review books as well as whatever else is on their mind. People are using their own recommendations and lists to tell something about themselves on a site they themselves created, as opposed to Amazon.com using customer reviews and enforce their own brand.
Combine these blogs with Froogle and Amazon.com should be worried.