I don't think people who say that this UI is nothing new or that Harmattan had it better realize how the usability of your device will change if you can interact with 3,4 or 5 applications at the same time on the same screen without additional touches/clicks/key-presses/whatever - even desktop operating systems don't have that solved properly, so IMO, if Jolla pulls this off, its definitely groundbreaking.

The only concern is if they make it fast, fluid and shiny enough to attract the masses.