You have to make concessions for a lot of old games. They're super important and influential, sure, but time is harsh to many of them. So perhaps the most impressive thing about Super Mario Bros. is how playable it is. You can jump around and play Super Mario Bros. and it feels fine and functional. Sure, it's not good. Level design is pretty awful, look at the multiple levels that are just a mostly straight walk to the goal with fish jumping up at you but hey, it works, and it has that instantly iconic quality. Y'know. Mario.