For me, the biggest hurdle that Crimson Skies would have to overcome was the balance between a realistic flight model and exciting gameplay. Make it too much like real flight, and the gunplay gets overshadowed. Make the flight model too simple, and no one will enjoy it. The is a fine balance indeed, and difficult to achieve. Next to that, the other hurdles are rather minimal, but include such things as complete and useful control schemes, plot development, and assorted other hurdles that are pretty much the same for any other type of game.
Single player is absolutely fabulous.
We are looking at an alternate US history, and with the emphasis on air traffic in this timeline, the plane evolved instead of the car. So you get downright freaky looking planes. Okay, that was a tangent, sorry. As I was saying, the Single player is highly mission driven, as you would expect. No they are always multipart, you always have stuff popping up in the middle of your missions, and they have a lot of variety, which is their main draw for me.