Keeping in mind how ridiculously primitive and cumbersome Atari 2600 was to program on, I think it's actually remarkable how functional E.T. is (adding in the time pressure and all).
I never got around to read Racing the Beam [1] but Retro Game Mechanics Explained on YouTube has a good demonstration [2] on how the graphics programming seemed like a nightmare compared to later computers and game consoles. Just positioning things on screen required calculating machine cycles of other preceeding instructions. Refactoring must've been fun.