Amazon's UX on Prime Instant on the PC is so terrible, their discoverability on ANY platform is awful, and they have restricted HD streaming of Prime content (i.e., stuff you don't have to buy) to non-PC platforms only. And I've had far more problems with buffering on Prime Instant on Xbox 360 than I have ever had with Netflix, despite watching roughly five times as much content on Netflix. It's ridiculous.

Their music offering looks like more of the same. Lower selection, poor discoverability, a second-class UI on PC. I really wish Amazon would stop re-purposing their storefront UI for every product they can, and actually design a decent UI for their various products.