The visuals are very primitive, but man there is so much personality here. Many games like to think they are classic, but I do not think you get much more classic than Super Mario Bros. Even the final castle level which has to be completed in a specific way was a big deal back when this game was first released.
After this, the game ramps up the difficulty and requires a great deal of skill to get to the end. What I think this game does very, very well is make it so that pretty much any gamer, no matter their skill level can get through the first world. The fact of the matter is, Super Mario Bros is still a joy to play. Yes, Super Mario Bros 3, Super Mario World and the various New Super Mario Bros games have taken this formula and improved on it in every way you could imagine.
I say that because the original Super Mario Bros has aged like a fine wine. It is crazy that we are not all that far off from this game being 40 years old as I write this.