Friends become enemies constantly, and even though you're almost immediately revived after being forced to fall to your death or pushed into the path of one of the Hammer Bros' errant projectiles, the pain of deception and backstabbery lingers long after (as does losing one of your limited pool of lives). Sure, you can play cooperatively like nice, civilized citizens of the Mushroom Kingdom, but where's the fun in that?
The new suits and abilities of Mario and co. really come into their own in the multiplayer. Iceballs thrown at foes turn them into blocks, which you can then hop on to grab that nigh-reachable set of coins ... or you can just be a jerk and throw the blocks at your teammates to stun them. Or you can use the Penguin suit and slide into a buddy, forcing him or her down a dark hole, never to be heard from again. Combine all of these crazy abilities with a simple, two-button control scheme and you have a unique, ridiculous experience almost every time.
While those two-button controls work great, Nintendo needlessly chose to add motion controls for some silly things. For example, you have to shake the Wiimote to raise yourself in the air with the Propeller suit or when you want to pick up other characters. It just feels unneccesarily complicated when you could just press a button. And sorry, folks - no Classic Controller support.
Though the motion controlled bits are annoying, there's not much in the game I'd describe as difficult, despite what Shiggy said. The inclusion of Super Mario World's mid-level checkpoints for stages that, to be honest, only take about five to ten minutes to finish is a little too forgiving. I know Nintendo wanted to eradicate the frustration of having to go through an entire stage again (all 5-10 minutes of it!) should the player perish, I get that, but it's gone way too far, taking the game from accessible to easy. Even though it played like the Mario games of old, it never even approached their challenge.
But how important is that, really, when New Super Mario Bros. Wii is so much fun through and through. Lack of difficulty bugs me, but it does mean that anyone can pick it up and immediately get into it, and anyone can do well in the game. Sure, it sucks there's no online connectivity for the multiplayer, but I know I would miss seeing the expression on friends' faces when I penguin-slid them into a nearby abyss. Even without online, this as an extremely smart rethinking of a tried-and-true formula and with at least one friend (or three), you're going to find New Super Mario Bros. is well-deserving of your hard-earned coins.