It might not have the notoriety of being the first 3D Zelda, or the wide-open epicness of Ocarina of Time and Breath of the Wild. But what The Legend of Zelda: Majora's Mask does different is where its strengths lay.
Rather than taking the straightforward adventure route of prior Zelda games, Majora's gameplay loop is a loop in the mightiest sense. Link is in an alternate reality where the moon is about to fall, and without the resources necessary to stop it, he must relive the same three in-game days (about 1 hour in real-time) over and over, each time gaining not only more tools, but also more knowledge about all of the people affected by the coming disaster. This knowledge gets logged in a notebook, eventually showing Link how to bring closure each of their problems by being in the right place at the right time.
These problems range from shopkeepers not being able to stock a certain item to reuniting a soon-to-be-wed couple to healing lost souls with a song. Each time Link helps someone out, he's rewarded with a mask that gives him a new ability. Compared to Link's main set of tools, most of these are minor abilities such as being able to get more information from people or being able to speak to certain animals and monsters.
However, three of the masks allow Link to transform into members of each of the regional races, with each one COMPLETELY changing the gameplay with a wholly different body type and moveset.
The two words that best sum up Majora's Mask are "dense" and "variety". While the world is easily the smallest of the major console Zeldas, every single inch is filled with unique and worthwhile tasks to accomplish. Only four main dungeons have been compensated for with an extensive leadup to each one, as well as an enormous quantity of sidequests that are really more key to the flow of the game than the main path. Add in how the gameplay instantly turns on its head each time you put on a transformation mask plus the deep personalities of the multitude of NPCs, and the lack of physical scale just outright vanishes.
Some people find the game's timer stressful. But as long as you only tackle one major task per cycle, the timer rarely feels like anything other than a time-of-day indicator. This does put more onus on the player to pay attention rather than enjoy the sights, but as the sights are intended to give off a vibe of weariness rather than field-and-lake style glory, I'd say it's not an unreasonable ask.
Getting back to the meat and potatoes of Zelda, the first three dungeons are all solidly designed (though like Ocarina of Time, the water temple is mazey and demands a strategy guide to prevent the onset of insanity) with an optional objective of finding Stray Fairies for major prizes, but they lack the atmosphere of the overworld. The BIG exception is the last one, the legendary Stone Tower Temple, which I (and others) consider to be the best dungeon in all of 3D Zelda. Not only is the atmosphere and variety of rooms and enemies superb, the central mechanic of turning the entire dungeon upside down is like two dungeons in one! Even the music changes!
The soundtrack is my personal favorite Zelda soundtrack. While a lot of tunes from Ocarina of Time are reused, it just makes the whole package sweeter. Including a full slate of new ocarina songs, the new musical additions have a similar structure to OOT's, but are more dark and dissonant. It's a fair step removed from Koji Kondo's usual upbeat style. Newcomer Toru Minegishi, who went on to be the lead composer of Twilight Princess and Splatoon, also helps out with some awesome battle music.
And as for that atmosphere I mentioned, it's the BEST aspect of Majora's Mask, as well as what it has the MOST of. The world is pretty at times, but is drenched in unease, and a layer of sadness consistently seeps through its facade. A masterfully realized trifecta of writing, visual effects, and soundtrack come together to create an experience that is constantly entrancing and affecting.
And along with everything else, Majora is one of the best looking games on the Nintendo 64. With the mandatory extra RAM of the Expansion Pak, Majora exceeds Ocarina of Time's visuals throughout with brighter colours, more details, a killer blur motion effect, and a more advanced lighting engine that is central to the mood of the world. Scenes are bathed in uneasy purples, yellows, golds, and pastels; characters are gently lit by flickering torches; and weapons such as the Bombs and Light Arrows fill up rooms with searing lightbursts.
The only truly negative opinion I have about MM is that it's just a bit easy. However, this slight fault in gameplay is overcome with ease by the outstanding presentation.