This Netflix blog entry
on the Win8 app says:
Video plays smoothly in full HD (when available). By bypassing browser plug-ins, we were able to make our video player more efficient. This results in smoother playback with less strain on your PC—and that means less fan noise and more movies on a single charge.
This implies that they couldn't manage 1080p (aka "full HD") in a browser player, though YouTube and VUDU have done it (their YouTube video on the Win8 app in that blog entry can be displayed in 1080p
They just posted a tech blog entry
about their work on replacing the Silverlight player with an HTML5 one yesterday. Right now they only have it working on the Samsung Chromebook.
One thing that the Win8 app does that I haven't seen done in a web app is bitstream Dolby sound (DD+ from Netflix titles which feature digital surround) on PCs which feature HDMI sound out. Perhaps there'll be an W3C HTML5 "Premium Extension" which will make that possible from a web app in the future.