It uses Windows 8.1 with bing. There already is another Intel stick released (in cooperation with Intel) with the same specifications. Windows 8.1 with bing gives OEMs the possibility to use Windows at no costs. The stick will be released with Internet Explorer with bing on it. Ofcourse as a user you are able to uninstall explorer and use Chrome for example, but OEMs are not allowed to do that (they are allowed to do that with the normal Windows, which also ships with bing).
This makes a full Windows available, which makes it possible to load Chrome as external HTML plugin, instead of the Adobe AIR plugin (something many users would love, according to the forum topics). Theoretical it should even be possible to use live TV on this stick, although I think this will be to CPU intensive.
I for one, am seriously considering these devices, because I really don't like the Android player as it is right now. It's just not stable and waaaaaaay to buggy to be an Enterprise solution (more something for hobbyists). Besides, Windows offers more possibilities like RDM.
I am wondering what kind of BIOS / UEFI this stick has. But, as with the Android sticks, I believe that around the start of Q3 there will be loads of these sticks available.