Free 2-day shipping was an absolute game changer. Customer reviews was also a huge competitive advantage that brick-and-mortar retailers lacked since it let you

Free 2-day shipping was an absolute game changer. Customer reviews was also a huge competitive advantage that brick-and-mortar retailers lacked since it let you quickly identify which specific product was most popular for what you were looking for: much better than asking a store rep for their recommendation. I'd really be interested in a graph that showed some growth metric like revenue or number of orders plotted against a timeline of when Amazon introduced certain features. I remember seeing one for Facebook that was really illuminating.