Making a 2D physics engine, especially with diagonal terrain, is very difficult.
Making a 3D physics engine is impossible without advanced knowledge of trigonometry.
Making a game with pathfinding is extremely difficult.
Designing a game without one of those things involved is hard.
Anything else? I remember a good few people having trouble with minimaps, and I haven’t tried making one, but I’m pretty sure it’s not hard.
Also, I think people should pool some ideas in here that DON’T involve those difficult things I mentioned… I might even end up turning some ideas into games (and I encourage others to do the same), because I feel I haven’t been anywhere near as productive as I’d like to have been over the last few years.