Interesting video:
A little longish, but worth a look; it's got some talk from a recent dev meeting and some more game footage.
The devs talk about the flight modeling, which sounds really complex and fascinating - basically, this new simulator breaks down each plane into a thousand or so tiny surfaces and calculates air interaction with each surface independently. This allows them to genuinely simulate things like aerodynamic forces affecting different areas of the plane differently at certain attitudes, making the handling extremely realistic. Contrast this with the flight modeling in Flight Simulator X, where each plane has one single model file - basically one "data point" - that the wind interacts with in a basic way, and the effects of differential aerodynamic forces in flight weren't actually simulated but rather were just programmed to happen under certain conditions.
They also talk a little bit about the world simulation, but not enough for me to really understand it what's going on with it. It sounds like it's not built out of individually-modeled assets, but procedurally-generated ones based on satellite and aerial photography data. I'm not sure of the specifics, but if the video clips are anything to go by, whatever the heck they're doing looks amazing. One of the things I always liked about the Flight Simulator titles is that you could fly over your home city, look down, and actually recognize the layout of the main streets and especially state and interstate highways. Only big cities got handcrafted landmarks of course, the rest usually being just procedural placement of some stock building models and texture tiles; but you could still orient yourself. If there was a river valley snaking through your town, that river valley was there and the course was generally accurate to within a certain scale. If there's a bridge crossing the river in RL, it would be replaced by a "stock" bridge asset (unless it was a famous bridge) but it would be there. By contrast the only moderately up-to-date commercial flight sim that's out there, X-Plane, uses a "plausible world" approach which will generate a "city" where a city ought to be in the real-world that certainly looks quite nice, but does not really visually match up in any way with what you'd actually see flying over that city for real unless, again, you go and download a handmade scenery file that the community has produced.
One thing I don't see enough evidence to judge yet, though, is whether the new MSFS will have contoured airports. In FSX, as in all previous MS titles, airports were highly accurate insofar as airport runway/taxiway layout, but they were all necessarily flat tiles - the entire airport was at the airport's official elevation as listed in the RL directory. In RL though, airports and runways aren't always 100% flat. This was something X-Plane beat FSX at....although on the other hand, X-Plane didn't have accurate airport layouts for any but a few major/"popular" airports; if you wanted to fly out of some little out-of-the-way airport that you happen to know, well there'd be a runway there but no taxiways or buildings.