One thing that is different is the lack of orbital decay. In kerbal, if you get above 68km, you will stay there forever. In real life, even extremely high orbits are subject to decay, since our atmosphere is much more complicated than KP can model.
Things like the ISS have to constantly make adjustments to stay in orbit. Things like solar flares have a tendency to spew atmospheric particles up into higher orbits and make orbiting objects slow down.
I think the game is definitely much easier with its simplified atmospheric model, and I for one am glad to not have to worry about my orbit changing once I achieve one.