A cool little webpage here shows the relative quality difference between a standard DVD and a high definition disc (in this case, HD-DVD). Look at Naomi Watt’s face in particular.

I’m noticing in a lot of side-by-side comparison shots (examples here) the high-definition image is so detailed that you notice mistakes in costumes and make-up (hey, Mystique’s ridges are peeling off!), and contact lenses in period dramas become a real concern.