Heres a few answers. Along with the footwork you mentioned true all fielders are trained at grounders but not all infielders have to stay on their bag like 1st does to reach and extend for bad throws. More plays at first then other infield positions. Now include small ball awareness playing up and back. Back pick-offs from catchers. Coaches who have 1st take cut offs. Playing foul territory with fence awareness.(other than bag footwork, I'm honestly not sure what special 1B skills even exist).
Others are talking about shortstop being the most difficult but i would say second base has to cover the same field territory including more small ball awareness and movement covering first base and pick offs from catchers. Second base also covers backpicks at second base. Shortstop is not as involved at the bases.