If this isn't a case of "Night of the Living Thread," I don't know what is. Quite frankly, I'm not sure why this hasn't been an ongoing debate...
Either way, its something that cannot be easily defined, as it varies largely from person-to-person. While I may find the '67 Corvette to be one of the most-beautiful cars to ever drive down any kind of road, I've known quite a few people who didn't like them one bit.
The same thing can be said of my love for VanGogh's work, particularly his "Starry Night" painting, which is probably my most-favorite painting of all-time. But, there are a lot of people who don't like it either, because its too dark, or odd, or whatever.
I dunno. Color, porportion, or just the way the object/person/place (whatever) looks, acts, feels, smells, etc like plays a large role with anyone as to how they are going to feel about it.
True beauty is in the eye of the beholder, and what had once considered to be the standards of beauty have either died, been destroyed, or have largely become irrelevant by comparison to today's standards. Its something that will change with every generation, for as long as there is another generation to continue the trend.