First off, I'd like to acknowledge that in a just world this game would be reviewing *me* instead of this way around, but that's just not how the cookie crumbles.
I was inspired to finally give this game a full run-through during my playthrough of Super Mario Maker for 3DS, especially with a lot of that game's levels referencing this one's. For completion's sake, I went ahead and completed the game twice, once in new game plus (when it replaces all of the Goombas with Buzy Beetles and increases the game speed).
A game like this is really hard to analyze from a modern perspective. Something like this that's a trendsetter and genre definer, but a game that still has a decent set of obvious flaws that would be ironed out over the course of this game's sequels. Is it fair to apply modern-era game analysis to something like this that pretty much predates the notion of such things? Unfortunately, I lack these answers, but I'm going to attempt to give this game its flowers while also discussing the things that it could definitely do better.
As the inventor of the modern platformer genre as we know it, this game introduced many things that would go on to become series staples. The momentum based movement and smoothly accelerating and decelerating jumps are incredibly intuitive in comparison to the platfomers that released before this game, although there are a few issues that need to be ironed out to reach the modern state of movement. First off, you have little (not none like many games of this era) control over your jump once you commit to it, which makes it very difficult to fine-tune your movements in midair and led me to a lot of (what made me feel like) unfair jumps throughout the course of the game. You also feel a lot heavier in this game when falling than in lots of other platformers. These things combined, while sounding small, really change how you use your muscle memory when playing this game, and again leads to a lot of unfortunate deaths, and this overall feeling of primitiveness and stiffness when trying to play the game today.
Thankfully, this game excels in a lot of aspects. I would say that the game's difficulty is surprisingly reasonable, especially for the era, and tons of these stages would go on to be remixed, mashed-up, and used for inspiration in tons of games throughout the past few decades. There's a reason this game's 1-1 is so iconic - it does so much well. The level design actually gets better in my opinion as the game goes on, with it becoming somewhat easier and seemingly built for speeding through, with well placed jumps and enemies that give you a chance to flow through the stage like butter. Not having any major collectables does this a favor as well, not asking you to sacrifice your speed to hunt like the newer games in this series do. If you do decide to slow down and explore though you will be rewarded with extra lives and secret, optional areas (as you should be).
Only a handful of levels rubbed me the wrong way, with the maze stages that take place in the castle being particularly annoying. The Hammer Bros. are also absolutely not making it through the pearly gates. A couple of other random difficulty spikes definitely help to show this game's age, but overall the stages are well designed and flow well together. Another strange note is that the new game plus mode seems to re-use certain levels instead of just porting each level directly in order. Either the stages are just very similar and I didn't notice the first time around - or it's a very strange choice to do here. (It is also absolute cop behavior to downgrade you to small Mario if you get hit with a Fire Flower - so glad we changed that).
The game excels in the visual and audio departments. This game's entire soundtrack and sound effects still ring throughout the series to this day, with the obvious standout being the main theme of the game. The visuals are iconic, and even though the strange ground texture doesn't make much sense, the ? blocks, brick blocks, warp pipes, and other paraphernalia from this game ended up being timeless symbols of gaming as a whole. Some of the enemies do look a bit strange, with the Koopa Troopas and Bowser specifically getting big redesigns later in the series, but even then their appearance doesn't look off, if they would have stuck with these designs I could absolutely see them all the way into the modern day.
So, I mean, yeah, Super Mario Bros. is good. Of course it is, we all knew this and you didn't need me to tell you, but it's a game that represents a unique shift towards the future. A game still learning from the mistakes of those who had come before it, but one that paved the way for modern gaming arguably more than any other game released - if nothing else platformers for sure. It's a classic, a game that holds up better than pretty much anything else from that era - but still with some glaring flaws to watch out for. If you're looking for a chill time I would recommend using save states (either for each world or at each level) to emulate that modern game save system and to make the game feel less dated and punishing.