Just a quick note to let the media development community know that we don't support the "opacity" property on the MediaElement. The current documentation (as of the date of this writing) does not detail this limitation of the MediaElement control in the Windows store app (WSA) development environment (formerly Metro). I am working with the documentation folks to get this updated and hopefully we will see an update before too long.
The reason for this limitation is due to the way that the Media Engine (the underlying component that drives the MediaElement) is being used by the XAML framework. There are a number of modes that the Media Engine can use to deliver frames to a rich compositor such as the XAML framework. The Media Element can work in either frame-server or rendering mode. Frame-server mode allows the Media Engine to offer up individual frames that can be used in a composition chain. Rendering mode allows the Media Engine to render directly on a "DirectComposition" (DComp) surface provided by the application. In the current XAML framework implementation we use DComp for the rendering.
DComp has the benefit of being hardware accelerated and is integrated directly into the DWM. This gives the MediaElement the best possible video playback performance and allows it to have a frame-rate independent of its XAML host.