The orbital mechanics, aerodynamics all of it, how real is it?
The orbital mechanics are very good, but not ultimatively real. It is maybe 1/50th of the accuracy of professional simulations per simulated day, but Orbiter also does this in real-time - it would be very painful for example to include all 2000 measured J-terms for non-spherical gravity, including the seasonal effects (GOCE even measured falling leaves in the autumn).
Orbiter does also not have variations in solar activity and its effects on Earths atmosphere. But thats OK, since scientists still have no good model of the solar activity cycles.
And Orbiter does not mind relativity... but thats a rather small factor unless you are close to the sun.
The aerodynamics depend on the spacecraft. The more the add-on does itself, the more accurate you can get. Especially, this also includes satellite aerodynamics. Practically Orbiter does everything it can do there satisfactory. It helps you a lot and is not standing in the way for more advanced stuff. A better general purpose API for defining aerodynamics would be nice, but isn't necessary. The rest is the job of add-on developers and framework developers.
The lack of weather and wind is a small factor... but its a spaceflight simulator. Space weather would be nice, but thats a job for add-on developers again.