When a product has a large number of reviews--both positive and negative--it can be time-consuming to read through them all to understand the range of opinions on a product. Amazon helps its users by highlighting the most useful positive and negative reviews (click to see full-size image):
This approach, of course, highlights only a small subset of the review content. But even when users read an entire set of reviews thoroughly, they are getting an unscientific sampling of opinion. Comparing a positive versus a negative review is likely an effective shortcut for those who don't have the time or inclination to read every review.
