I'm writing this fresh off of beating this game for the first time.
This game, simply put, is a masterpiece. The quirkiness and irreverent humor that made Earthbound such a hit has been mixed with an equal proportion of raw emotion, to the point that there are certain scenes where you don't know WHETHER to laugh or cry.
But you've heard a lot about the storyline of this game, probably. You may have even had it spoiled for you, by Brawl trophies, or simple curiosity. Speaking as someone else who has had this game relentlessly spoiled for me, allow me to say that the emotional impact of the game is not cushioned at all by knowing what's about to happen. I learned essentially everything important about the plot long before I played it, and I was still brought to tears by this game.
As mechanics go, this game is excellently executed. Earthbound was hampered by a clunky interface that forced you to select actions from a menu even when attempting to open a present or talk to a person; here, there is no such limitation. It's also able to avoid being a total grindfest; if you do all the game tasks without taking any time out for level grinding, the game is challenging but no less fun for it. This is helped by the fact that when you die, you merely return to the last save point, instead of needing to load that save again; thus, you keep all your experience, levels, new moves learned, and items picked up since then.
The game's chock-full of easter eggs, hidden secrets, and amusing diversions, as well. There's an item-aging mechanic (Fresh Milk becomes Rotten Milk, which then becomes Yogurt) and, of course, like all Mother
games, the ability to sneak up on your opponent's back to get in a free hit. There's also an interesting battle mechanic; by pressing the attack button to a certain beat, different for every enemy, you can get in extra hits. It's hard to get more than three at a time, but potentially 16 hits per turn are possible! Even if you can't get it down, though, you won't have a significantly harder time with enemies.
The finale of the game is one of the most rawly emotional, heartrending scenes ever programmed. Altogether, this game is surely Shigesato Itoi's magnum opus.