DRM
What I find crazy is that none of these "Smart" dongles (as far as I am aware) allow users to access their "legal" movie libraries, such as Ultraviolet or Digital Copy. If something like the Chromecast could play content from these "digital lockers" it would be a huge leap forward for both the technology and the services. Instead, both are crippled by the restrictions of DRM and forcing the average Joe to ignore the potential of these devices, and the tech savvy down the path of creating their own DRM-Free media to use with flexible media software such as XBMC or Plex. You could give these things away but without access to all of your legal media, the novelty will soon wear off.
iPlayer is a DRM solution done right - it is restricted but open enough to be pimped onto every device going, accessible to all. Ultraviolet for example is far too locked down to be of any practical use. If the Chromecast could tap into the Play Store to stream purchased content, or even on a rental basis, it would be a winner.