Majora's Mask is Everything that Ocarina of Time Lacks
Majora's Mask starts off with Link and Epona, slowly walking through a foggy forest. They come across the Skull Kid (from OoT), who has become possessed by a mysterious mask (Majora's Mask). Link gets knocked off his horse and when he wakes up, all his items, including the Ocarina and his horse are missing. He goes out to search for them, only to fall into a deep hole which brings him to an alternate universe, known as Termina.
Once he arrives in Termina, he meets The Happy Mask Salesman, who has lost the Majora's Mask and he asks Link to help him return it, for the mask has evil, destructive powers that if in the wrong hands, could be disastrous. Link soon discovers that the people of Termina are in despair. The moon is slowly falling and will come crashing to the Earth within 3 days, destroying everything in its path.
There's a time limit which adds a lot to the game. After 3 days, it's game over. However, at any point you can play the song of time to send you back to the very first day, repeating the same 3 day cycle. Everyone in the game has their own routine. The longer you play the game, the more you'll memorize the daily cycles of each character and where each character is at all times. You can however cause different events which will disrupt the cycle, creating a whole new set of events.
Majora's Mask's main plot often is criticized for being too weird and quirky, while I personally think this makes the game much more original and fascinating. The dark feel of the game makes it more emotional and captures the essence of disparity as the moon slowly makes its way down to the land of Termina. It draws you (well, me anyways) in more than any other Zelda game in existence. The dark atmosphere also makes the game more mysterious than other games.
The aspect of time also allows for more sidequests. The sidequests are synchronized well with time and make the game feel more open and free instead of orderly and linear which is a major plus. Although the game doesn't have the same quantity of dungeons or required quests as its predecessor, OoT, it makes up for it in the quality of gameplay and the tremendous amount of interesting sidequests. The sidequests are completely optional though.
One of the most notable features are the masks. There are 24 different masks to collect. Some are optional and give you powers such as increased speed, mind reading, or invisibility while others can completely transform you into Deku Scrubs, Gorons or Zoras. These new forms can give you many new abilities such as flight, swimming like a dolphin, and rolling at incredible speeds.
Technically speaking, MM improved upon graphics, with a few details enhanced but nothing really that will jump out at you as being different. There is nowhere in the game that is not 3D, unlike places such as Market or the outside of the Temple of Time in OoT.
I have never in my life played such an amazing game. To call it anything other than a masterpiece would be an understatement. Majora's Mask is one of the most daring, yet under-appreciated video games of all time. It's a shame that the game has not been embraced the same as Ocarina of Time. It improved on pretty much every aspect, and added an enormous amount of personality and originality unparalleled by any other Zelda game. I STRONGLY recommend this to any gamer.