Taking a wild guess here, the issue sounds like a GPU hang or crash. This gives the illusion of total unresponsiveness when only the display is not updated anymore. It would be interesting to see if networking still works in that frozen state. The CPU seems to be unaffected from the looks of it (sound decoding and playback works).
Agreed — and be rather unsurprising given the patchy nature of video driver quality. It'd be really easy to believe that Metal 2 is putting more pressure on a previously less used code-path and is flushing out bugs or hardware issues.
