That is hard to answer without getting biased. Right now, most of the stuff seems trivial and natural - rockets, spaceplanes, SSTOs, rendezvous, docking, transfers, gravity assists*, precision landings, you name it. But I knew a complete jack about orbital mechanics or rocketry when I started playing KSP, so EVERYTHING was new and hard. It was like learning how to waddle, then walk, run, jump... I'm pretty sure there wasn't a learning wall, there were several of them - it was more like learning stairs. The game itself is not hard to play once you know what you are doing, but figuring out exactly what to do for a complete newbie to the field is very hard. I feel no shame in admitting that most of my KSP journey was babysitted by various tutorials from people who have figured the stuff out before me. And I have no doubt that there is going to be a lot to learn in the future (especially new aerodynamics and re entry shock heating with 1.0, I have very little experience with FAR/NEAR and DR). *I still need a calculator when going for multiple slingshots, and that is probably never going to change