Checkpoint system + lack of joystick support + occasional crash to desktop + punishing difficulty = frustration
Another thing that annoyed me was no rudder. Yep thats right, they decided to build a flight game but ignore how an airplane is supposed to fly. Rather you have to preform an awkward barrel roll to get orientated on your target and this can be very frustrating especially in later levels.
Another annoyance was the addition of a check point system rather then a permanent save option. This really pisses me off when developers do this on the PC. It makes the game feel like a crappy port from a console game. And if this wasn't bad enough the game, even after the update crashed to desktop on me every once in awhile. If that happens you are screwed, as the game does not permanently save the checkpoints. You gotta start at the beginning of the mission. Nothing is more annoying then playing a long frustrating mission, with poor controls, and after an hour finally making your way to the last objective only to have the game crash and you losing all your progress.
This could have been a great game, if it wasn't put together so half-assed. It has great graphics, an interesting storyline and plenty of variety...but all that does not make up for the frustration and anger it causes.