You should know that with this update, there is some steps made that makes XAML apps much much more powerful for Desktop/Business Scenarios. First is that they can run windowed/or snapped like before. Second, sideloaded apps get full access to desktop apis using some kind of brokered process, but still pretty seamless for developers. Third is the sideloading deployment is now much easier.
so i personally don't really see the need for WPF anymore(which is still supported and works pretty well), because there is now FULL desktop support actually. You just have to gather your informations. See: https://channel9.msdn.com/Events/Build/2014/2-515
it's cool, pretty easy and lightweight it seems. If they put decent tooling in, it might get used by People.
Though when we're going down the js as an assemblylanguage road, I'd probably choose C# (e.g. Script#) or even cooler, F# (Websharper). or imagine being able to cross-compile your portable class libs down to js..
But what I'm also waiting for is for something Like Google Native Client as a (real) assembly on the web to take over, we can't be stuck with JS as the ONLY language forever.
If the app gets hibernated(or shutdown by os) while playing a video, it gets resetted. also, background playing the audio would be nice. It's more a general os multitasking issue when playing media(a video should not essentially stop when navigating away).