The one thing Orbiter is lacking in is that all the terrain is just a flat 2d landscape, and that's the one thing that urks me about it, at the same time though you can't blame it for it. generating terrain, would probably be probably be pretty laggy, and bumb mapping would take ages, although I'd like to see it done. Another thing is it's a lot less user friendly. You can set up a manuever node and see if you'll get an intercept. Plus sometimes the MFD's can take a bit of effort to learn. And one final negative, is it's lack of collision. The community barely stay's alive from what I see :/. I've often thought about contacting the developer to see if he plans to do anything with it in the future to improve the simulator. In the end though the challenge and satisfaction orbiter bring is excellent and if your willing to look past it's ugliness (though the planets look excellent in space) you'll enjoy it immensly P.S Sorry for any poor grammar in this post. Feel free to correct me.