Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
Legend of Zelda: Mahjora's Mask used to answer the old question "Just how much extra stuff can you put in an action adventure game, while not losing the main adventure almost completely. You know, like Morrowind. As is befit, this game only has one plot... but dozens of small stories. And as such, it is, I think, a better game than Ocarina of time, a comparison that must inevitably be made.
But I'm getting a bit ahead of myself. In Mahjora's Mask, our oh-so-famous hero Link has travelled away from Hyrule in search of a friend. I'm not sure why, because he'd already made plenty of friends in Hyrule itself. But be that as it may, he eventually ends up in Termina, where it appears that the Moon is in fact falling down, in exactly 72 hours, but where one hour is translated into one minute for gameplay purpouses. Of course, not everyone believes that big ol' chunck will really fall down, which goes to show how strong the power of denial can be. Just like the real world, in other words.
And that is when the concept of time in gameplay kicks in. Time is actually moving, and as such, an NPC's location may change from time to time. A tower that people started building on day one is nearly finished on day three. And time always moves on, except when you're talking to someone and pausing the game. But to make sure you don't end up squished flatter than a pancake, your trusty, trusty ocarina will allow you to move back to the beginning of the cycle. You'll lose your money (unless you've deposited them in the bank, which is inexplicably immune to the effects of you time travelling), and minor items (i.e. anything that can be counted and found by destroying grass and pots and bushes); while major items are kept. It's a most interesting and original concept, and, I imagine, damn hard to execute well, because hardly anyone else has tried it afterwards. But it is done well here.
The other new concept are masks and transformations. Link actually starts out as a Deku Scrub, but will eventually grow back to human gain up to 4 different forms: The decu scrub is small and can glide in the air. The goron form is big, bulky, and can roll along the ground like a ball. And finally, the zora form can use electricity as an offencive shield, as well as swimming swiftly along the ocean.
And this is where the first comparison with OOT comes in mind: Vastly improved movement. The goron roll is great to get around the main field with, and would have been invaluable to the bigger, but emptier Hyryle Field. And the swimming, my gods, the swimming! Instead of the slow, clunky affair of a human in the ocean (not to mention the slow affair of a human in iron boots in the ocean....*sudder*), you can (and will) instead opt for the amazing swiftness of the zora elegantly going through the water like a dolphin. Including that you can jump out of water like one. And even ordinary Link's got an improvement with the bunny mask, which speeds him up with about 50%. I have spent hours and hours simply using all these improved movements on an open field or ocean, just for the sheer fun of it, eventually wearing out the analog stick on the controller.
The other masks are ranging from one-time-use masks in certain situations, or they can actually provide a difference in your main gameplay. A bomb mask will explode in your face (in case you should actually run out of bombs), the rabbit mask will improve your running, and the stone mask will camouflage you from (most) enemy eyes. And most masks will change the reactions of the NPCs you meet in some manner.
Speaking of which, the next step of improvement from OOT are the NPCs. THey are no longer merely the classical "the castle have many guards" source of information, they have lives and personalities. The captain of the guard is insisting to evacuate the town, while the chief of merchant won't stand for such a possible loss of money, as a festival is coming up. The runner of an inn not only has problems with her lost love, she also still has to run the inn on a day-to-day basis. While things are more traditional outside Termina City, inside, just about everyone has a story to tell. And rewards to follow if you help them resolve their problems, of course. Thankfully, you'll gain a notebook to keep track of not only who needs help, but more importantly -when- they need it. Which can range from anytime to only one minute during the entire 72-hour period. Like I said, you'll need that notebook.
And now we get back to my original point, on how much more things there are to do than in OOT. I'll illustrate this with our friendly, well-known, Heart Pieces. In OOT, you've got 36 of them, and about 30 are merely a matter of knowing where they are, with 6 of them requiring some extra skill, such as the arrow-shooting from horseback minigame. In MM, you have 52 pieces of hearts to collect, and you need to test your skills in about half of them. That's 20 more "skill" hearts than OOT. There are three very different shooting games, there's a swimming game, a sword game, jumping game, and plenty of other little uses of skill ranging from easy to extremely difficult. One could never have won those archery games with the unresponsive analog stick of the PS2 Dual Shock, that's for sure.
Another thing that really shows the difference is the central field. In OOT, it was used mainly to show what you could do with an N64. It was mostly empty, and you'd spend a lot of time merely running through it from one place to the other. Here, the big plain is smaller, the important town is in the middle of it, the scenery is more varying, and there's a lot more to do. And the new movement modes allow you to get from one end to the other much quicker, of course.
The music is... Well, I've never really been that fond of the "classic" overworld theme, preferring instead the plains in OOT that also changes pace depending on how much you're moving around, and whether any enemies are near. But there are certainly highlights here as well, and only one reuse of a theme from OOT. I'd call this one a draw.
If we're to get back to the main game, then the dungeon bosses have also improved a bit from you-know-which-game-I-mean. But the ending boss is... Well, to put it short, it's not Ganon. It's instead just the embodification of the powers of a mask (Mahjora's Mask, to be precise). And that, while more novel, can never be the same as Ganon. The main plot is also just not as... epic, really. Ganon taking over Hyrule is much better than what is essentially a moral on how not to lose your friends (you'll have to discover for yourself how a moon falling down and crushing everything can represent this), and that's a fact.
But that aside, this game is an improvement in just about every factor. Smoother graphics, tighter controls, cooler NPCs with actual personalities, a targeting fairy not 1/50th as annoying as Navi, a whole lot more to explore outside the main plot, vastly improved movement (especially in water), some strange and quirky humour, an interesting time concept... In short, it's a very, very good game with the good stuff overcoming the few moments of irritation with ease.