re: What is so good about Majora's Mask?

Because let's say you unlock a mask, and you reset, you keep the mask permanently.

Let's say you have to do a sequence to unlock a dungeon, once you complete the sequence you will unlock a song that will take you to the very mouth of the dungeon, so you reset, play the song, and boom you have 3 full game days to beat the dungeon.

If you beat a boss in a dungeon, you keep the item he gives you after a reset.

Basically anytime you achieve a milestone, you are given something that gives you permanent progress in a temporary world. So you're constantly advancing even though time is standing still. Really the beauty of the game.