The perfect game.
There is not a single bad thing to say about Super Mario Bros. There aren't many new good things to say about it, either, since literally everyone (and also their mother) has played the game over the years, and loved it. You don't have to try to explain why SMB is so brilliant; it just is. And everybody knows it already!
But... seriously, just why is Super Mario Bros. so good?
The answer lies in its design.
There's not a single line of code in Super Mario Bros. that is misplaced. It's all wired down to exact perfection. Mario (and Luigi) jump at just the right height. They move at just the right speed(s). Their fireballs move in just the right motions. Their abilities give them just the right amount of advantage. And regardless of your own skill at the game, you always feel like you're in complete and total control of your character. Mario moves, stops, jumps, throws fireballs, and speeds up the very millisecond you give him the command.
The ability to run faster by holding down B is a genius move, as it works as both a helper and a hindrance: you can move faster and jump longer distances, but Mario also loses traction and becomes harder to control. Thus, the different speeds must always be kept balanced, and players can't just run or slog through the whole game.
And the game has the design chops to prop up the programming. The enemies are all perfectly designed to fill a specific hierarchal role: Goombas are the non-threatening starter enemies, Koopas are the tougher Goombas, Pirana Plants are the ones you need a power-up to defeat, etc. The second a new player first engages with a newly-introduced enemy, they figure out the way to defeat it immediately- either by killing it, or being killed by it!
SMB's system of power-ups is wonderfully designed. The Super Mushroom makes you stronger, so you can withstand a hit and break blocks. This makes you more at ease, since you won't die instantly from every attack, but you still don't want to plow through uncaringly, because you don't want to lose the power. You need to have a Super Mushroom first if you want a Fire Flower, but if you get hit with the Fire Flower, then you're automatically sent straight back to the bottom. This way, the player is still very cautious with their behavior, even though they are at their most powerful, because they don't want to have to start all over.
The Starman is also very well crafted, as it manages to take an item that makes you literally invulnerable and still make the game challenging while using it. Although Mario is invincible, he has a limited time, so the player must use the time wisely. You've got to be quick, and get through as much of the level as you can- but Mario can still die from falling into pits, and carelessness will easily send you straight into one. So you have to be fast but still careful, or else you'll be killed and have grabbed the power-up for nothing.
Getting extra lives is a carefully, perfectly managed practice. Get 100 coins, get a life. You've got to keep an eye out for as many coins as you can find, so you get more lives. Simple as that, right? Oh ho, not so! For there are mushrooms, hidden away in the last places you'd expect, that give you another life absolutely free. Oh, and sometimes, completely out of nowhere, you'll get magic vines that send you up into Coin Heaven, where you get a beautiful ballin' bounty of coins! Along with the power-ups, the 1-ups are carefully placed to ensure that players don't play too quickly: they always want to be on the lookout, searching every nook and cranny for any opportunity to get more chances to play. And if you do run out of lives and are forced back to the title screen, there's still one last hope! Hold down the A button when you press Start, and you will be transported to the beginning of the World you lost on! I don't know if this was even intentional (they don't mention it in the instruction manual), but if it's a glitch, it's a very lucky glitch! It keeps players playing! If you get to the final boss and die, it doesn't sting as much if you only have to play through World 8 again, rather than the entire thing.
And the levels! Oh, the levels! 32 obstacle courses of pure, distilled goodness. The whole composition is great. The game elements are all placed in jut the right spots, and the challenge is perfectly exponential- there is a distinct progress from very easy to very hard, rather than the schizophrenic difficulty of some of SMB's lesser imitators. The enemies get tougher and more numerous, the pits get wider, the power-ups get scarcer, and the secrets get less and less common. The 8 worlds have just enough variety to keep things fresh: some levels are set in fields during the day, some in fields at night, some underground, some underwater, some high in the sky, and some in castles, and each level has you serenaded by Koji Kondo's earliest, most famous, and greatest compositions. I doubt there's anyone in the world who's never heard, "da-da da, da da DA!"
There are many games that are bigger than Super Mario Bros. There are many games that are better than Super Mario Bros. There are many games that are longer, prettier, cleverer, funnier, smarter, and cooler than Super Mario Bros. But Super Mario Bros. is long, and pretty, and clever, and funny, and smart, and and cool, and it sings it all without a single flat note.
And, thus, I have only one way to describe it: Perfect.