The screen. Clearly that is the thing which makes this announcement. For me, the 3:2 aspect ratio is so more reasonable for computers than 16:9. And having a zillion pixels is wonderful although my CAD package (TurboCAD) still doesn't deal well with the high DPI screen off the Surface Book, I'm sure it would look silly on this machine.
My experience with the Surface Book tells me that the PixelSense technology is really great for drawing. I have both it and the iPad Pro and not too surprising, at twice the cost, in my opinion the Surface Book's drawing experience is better than the iPad's. I base that opinion on precision of the drawing, expressiveness, and the response time.
Touch. Microsoft is really doubling down on the whole touching thing and so far Apple has stayed away from it with its compute platforms. That is both a strength and a weakness. The rest of the ecosystem doesn't always understand what to do, so you get controls that are too small to use your finger on sometimes, and odd sort of multi-monitor experiences where things appear on one screen and then when you resize them they jump to the other and try to adjust for "touchiness".
If the tools people can get their act together, and by that I mean the designer tools (I for one would love to see a schematic capture and board layout system that was touch enabled and pen enabled) then I think it is only good news for Microsoft, if they can't, then Apple will look really smart at not adopting a "gimmick".
Either way these things are hugely fun to use and play with.