It's got some better materials on the inside in places, adjustable bolsters on the front seats (unlike the US M5...
) and I believe some other standard equipment -- but mostly you're paying for the "style tax". The same thing happens on the Mercedes side. The CLS Class is essentially an E Class from a feature and even manufacturing standpoint, and it costs Mercedes roughly the same to build either one, but the CLS costs more because the market will pay more for a flashy coupe than a relatively more "pedestrian" sedan.
I personally would prefer the M5 even if the prices were the same because I think that while the M6 has some sporty, aggressive, and cool styling in certain areas, it doesn't have the integrated styling "flow" that works as a whole like the M5 does. I also think the interior of the M5 is far more welcoming, whereas the M6 is clearly designed to be a driver-only car. I sat in an M6 on the passenger side, and that beam cutting down from the dash to the console really makes the passenger feel cut off and even sort of trapped -- and that OCEAN of carbon fiber trim...
Not to mention that the rear seats of an M5 are actually usable, whereas the M6 has less rear seat room than even the 3 Series Coupe!