The level design in New Super Mario Bros. Wii is fantastic. The designers have taken the classic platforming of the series and added some twists and turns, some of them quite literal. Mario has to climb moving fences, and jump across them in Spider-man-like manner. Some platforms rock back and forth, requiring perfect timing to cross. Blowing sand geysers and foggy levels would simply not have been possible on the Nintendo consoles of old. The few motion control areas are well done, not frustrating at all, and seem natural, not tacked on. There is a satisfying mix of familiarity and innovation in New Super Mario Bros. Wii that makes it very compelling.
The biggest change for the series in New Super Mario Bros. Wii is the addition of four player co-op. Up to four players can grab a Wii-mote, turn it sidewise like an NES controller, and have at it. In order from 1 to 4, players control Mario, Luigi, Yellow Toad, and Blue Toad. Two Toads is a strange choice, since they look so much alike and there are many other characters in the Mario world that could have made an appearance. There is no online co-op, which is pretty much expected for a Wii title, but still a shame.
Unfortunately, the co-op in New Super Mario Bros. Wii is not particularly strong. That's not to say that playing with four people isn't enjoyable, because it can be from time to time. But the multiplayer experience is better described as competitive, not cooperative. As more players are added, the gameplay becomes more chaotic. Instead of becoming easier with more people, as you'd expect in a co-op game, it becomes more difficult, particularly when playing with those of different skill levels. It's clear that the designers wanted a good co-op experience for the game, and I appreciate that effort. But there several issues that encourage cutthroat play, instead of cooperation.
The biggest issue is the fact that players can bump, collide, and otherwise mess with each other. At first this doesn’t seem like a big deal, until a teammate nudges you off a pipe into a Koopa or off a platform into a pit. Worse yet, you can pick up another player, carry them around, and even throw them. Far too often, this totally wrecks the timing and precision required for a platformer of this nature, even when it's not intentional.
Here’s an example: Mario freezes a bad guy and hops over it while waiting for another enemy to come in range. Luigi runs over, stands on top of the frozen baddie, and tries to pick up the block of ice to toss it at that second enemy. However, Luigi is standing too close to Mario, so the green plumber ends up picking up his brother instead of the ice block and hurls him directly at the second enemy. While the player controlling Mario is busy yelling at Luigi, the iced enemy thaws out and kills him while he's distracted. Admittedly, this is an extreme (and funny) example. But after you lose your last life because of a similar situation like this, you’ll want to throw your Wii-mote (or your “partner”) across the room in frustration.