Atmosphere of the game itself. i mean, ambience. not the air thingy. visual of the game, description on the parts... etc. and everyone mentioned already but patched conics & other casual mechanics too. I never had any interest in astronomy neither rockets. So if KSP wasn't look sily as it does now but was serious space sim, i might be too intimidated and already gave up. but light-hearted side of KSP keeps me stick to it even if keep failling something. because of that, failure is a hllarious moment rather than stressful moment. top of that, it's still deep enough to feel that accomplished something awsome when you succeed!
