An innovation of FSX is that it does now model the world as round and has increased the altitude up to which you can now go.
This means you can indeed now fly realistically to any point on the Earth and has enable the Space Shuttle (seen on a recent thread) to be modeled, but the change in how things are done is what is slowing down development of add-on scenery and aircraft.
The main reason to buy FSX (apart from this) is that it models atmospheric effects much better. plus it has innovative support for scripted 'adventures' too, so things like thermal activity along with improved real-time downloads of real-world weather from meteorological stations is much improved. it is a much better 'flight simulator' than its predecessors.
It's a frame-rate hog though, that's for sure.
Chock