Just been posted on the Diablo Youtube page
Dude! I know what the comet is!
You see the angels fighting there and so on, Cain says "Justice shall fall on the world of men". In other words, Tyrael is cast out from the High Heavens. Tyrael, who is the angel of Justice, is the comet that is literally falling on the world of men! It makes sense!
It's been known for quite a long time that the comet is Tyrael.