The Legend of Zelda: Majora's Mask is a sort of warped reflection of its predecessor, Ocarina of Time. It shares many of the same items, art assets, and even NPC models, yet the structure and even gameplay are considerably different. Majora's Mask is an experimental game, one which brings a lot of new ideas to the Zelda series but doesn't always work as well as it might have.
The core of Majora's Mask lies in the collection of masks. This was a series of sidequests in Ocarina of Time, but here, the masks are a primary part of gameplay. In particular, three masks will let you turn into three non-human forms (Deku Scrub, Goron, and Zora), each with their own abilities and attack patterns. The game features fewer items than Ocarina did, but none are made redundant by a second world, and the masks make up for the lack. Nintendo did a good job of trimming the tools down to their essence. Most of the masks beyond the main three have limited use, though several are valuable additions to Link's arsenal.
To correspond with the masks, the game features far fewer dungeons than any previous Zelda game. In fact, there are only four traditional Zelda dungeons, though there is at least another dungeon's worth of content just to unlock each of these. Still, the dearth of primary dungeons can make the pacing of the game feel odd, and results in a truly ridiculous number of pieces of heart to collect.
The other core conceit of Majora's Mask is that the entire game takes place on a repeating, three-day cycle. You can reset the clock at any time, and indeed this is how you save, but items such as money and ammunition are ephemeral. Actions you've taken that affect the world are also reset each time you begin anew. This is an interesting system which has its good and bad points. Every townsperson acts on a strict schedule, and analyzing their actions, Groundhog Day-style, is key to finding a number of the masks. This works incredibly well. But the game also occasionally requires you to replay significant sections of it just to find a minor secret, which can be a slog. On the whole, the three-day cycle works rather well once you get used to it.
Where Majora's Mask starts to fall apart is in its emphasis on execution, particularly of platforming elements. The N64 and the Zelda interface are really not well designed for some of the things you have to do, such as rolling around at high speeds as a Goron. The challenge seems to come from the poor controls and camera more than actual gameplay, which is a problem. There are several sections that require significant backtracking as a result of anything less than perfect precision, and these get old fast.
How you feel about Majora's Mask will largely come down to how much you enjoy its innovative time and mask mechanics, weighed against how much you hate its rocky challenge curve. There is a great game hidden behind some rough edges, but it takes effort to find it.