Being based on open web standards and being open source itself means SproutCore will enable developers to develop cross platform applications without being tied to either a plugin architecture or its vendor.
It took me only a few minutes with the PaperCube example to see that, whilst it is a pretty example of a web-app, it’s still nowhere near a desktop experience.
I find it extremely ironic, after decades of people bashing the Mac for its single-button mouse, to see so much pressure for web apps which often abandon the right-button and often the scroll-wheel, for other than scrolling the page (if the app doesn’t fit). Google apps, such as maps, make good use of the right-click context menu as well as scroll-wheels. Another superb example is Kerio’s webmail interface.