what windows app exactly will people trade for few hours of battery life?

seriously, other than PC games, I don't know a single windows app I want to run on a tablet form factor.

Maybe not, but they designed it so that it's a tablet/ultrabook hybrid. Do your research on the couch in tablet format, then move to the kitchen table to write it up in Microsoft Word. Read and write emails during a bathroom break, then create an insightful business presentation in Microsoft Powerpoint (don't forget to wash your hands)!

So what will happen when you have Word open in "ultrabook"/desktop mode, but then start using the Surface in tablet mode? Is the same instance of Word still open? Is its interface the same? These are the type of things that MS hasn't proven themselves good at designing. MS tends to overcomplicate their UI (Ribbon etc) and in my opinion, a tablet tends to benefit best from focus and simplicity.