Coming off the back of Ocarina of Time, it took me a little while to get what this game was doing.
It starts out very slowly, and takes quite a while to get to any traditional Zelda gameplay. Zelda games up to this point had carved out a pretty particular niche for themselves, and this game differed so much from that in the first few hours that it might be argued that it doesn't qualify as a Zelda game.
What it does qualify as is a brilliant game. The entire experience feels more full than Ocarina. Every sidequest feels worth doing for its own sake, just to help people out. The dungeons, while fewer, utilise more interesting and unique mechanics.
The masks don't merely replace the functionality of the tunics from OoT, they supersede them. Each of the transformations have multiple different abilities, including major additions to movement. And that's only the three major masks.
The ocarina is (ironically) much more useful in MM, rupees actually feel useful and important (and sometimes, scarce), the world is filled with all sorts of interesting and fun challenges, etc.
All that is to say, this game is a major improvement on its predecessor. I don't know if I can think of a single thing that I like more about OoT.
I highly recommend this game to everyone. Possibly my favourite Zelda game so far.