They had initially charged $200 for it on contract when comparable phones existed for free on contract. (The Prime offer was not sufficient)
When the cost was removed, it was just another Android phone.
I think it's worth pointing out that too that while "just another Android phone" even lay consumers can see the differences between Amazon's Android shop (and the Fire platform) and Google Play, fairly easily. From a lay perspective where "sideload" is a meaningless made up word, Amazon's platform certainly looks more like an "Android knockoff" than "just another Android phone".
I am also curious on people's thoughts relating to the design chops at Amazon.