Compare the weight and bulk of a medium format system with a D800 and lenses, etc.
Second, compare the handling and ease of use.
And of course -- $20k to look at a Hasselblad behind door one, versus $3k. And have you priced Hasselblad lenses lately?
In the end, you might be able to tell the difference in an extremely large print, from extremely close. I don't know. I don't own one. But I'll bet Hasselblad is looking over its shoulder more frequently now that the D800 -- and dare I say the top-shelf Canons as well -- have arrived. The digital cameras in 35mm-sized bodies have closed the gap by a large leap! And I don't think Hasselblads shoot video yet either, do they?