I think part of it is the switches. They use Topre capacitive switches which have a very distinct key feeling than your average mechanical switches made by Cherry (and its clones) or Alps.

RealForce (the keyboard brand from Topre) now have their own line of capacitive keyboards, but a while back HHKB was _the_ capacitive keyboard, or at least around the people I know.

Which makes it very interesting that this new model is using Cherry-type linear switches instead of Topre.