I think its materials, and higher precision manufacturing. Beyond a certain point, however, the law of diminishing returns really comes into play, twice the price doesn't mean twice the goodness, especially at the upper levels, when you might triple or quadruple the price for a tiny fraction of a quality increase. Just find out what you need, and get the cheapest thing that satisfies that, you'll be just as happy, and have money left over for something else.
But anyway, in terms of the actual hubs, I think its largely materials. I've seen really cheap cups actually bend and deform, as opposed to 'wear out'. The steel was obviously very low quality, and not properly hardened. A better hub would have had better materials, and it wouldn't have collapsed like that. A better design, and higher precision manufacuring probably play into it, but it all starts with quality materials.
peace,
sam