It's not true that every game becomes fun with co-op and how it's done can differ wildly. A game built to be done by a single player can have multiplayer tacked on and it can be more fun or less fun but isn't required. With TriForce Heroes the single player was tacked on. It's a game that absolutely requires 2 other humans. It seems to me that Nintendo didn't like the idea of requiring multiplayer so they made them tack on a single player option but the game is clearly designed around 3 human players, which is what makes it so good. Other games don't require cooperation as much and TriForce Heroes takes a bold direction.See as someone who played Four Sword Adventures solo and loved every second of it (one of my favorite zeldas actually), requiring multiplayer for it to be fun is already a big negative. Every game becomes fun in co-op with friends.
But I'll be honest, it's hard to believe this game is that great of a coop game anyway when a fundamental mechanic (the totem) freezes the movement of one or both of the other players. Actually that just sounds like bad design for co-op.
The totem mechanic is brilliant. You have to coordinate each player to get things done. The bottom players can't perform actions but control movement and the top player can't aim but performs the actions. You really have to work together. The use of height in the game is probably one of the best uses of the 3DS' stereoscopic 3D in a practical application.