New Super Mario Bros. U doesn't work cooperatively. When you throw multiple players into a tightly structured world, with enemies jumping around and intricately placed platforms floating just out of easy reach, only bad things can happen. Mario and Toad might both lunge for a power-up at the same time and collide in midair, and the fungal one gets killed by a passing goomba. Or maybe Mario accidentally jumps into Luigi from below, launches the younger brother unexpectedly into the air, and then sends the green guy sailing into the abyss.
New Super Mario Bros. U doesn't work cooperatively because there are no co-op mechanics. Even two equally talented and willing players will inevitable kill each other, no matter how much they try not to, because that's the nature of trying to harmoniously leap around an ever-changing environment.
What NSMBU does do, maybe exceptionally well, is allow particularly malevolent players to cause untold grief to their friends and frenemies. This concept is nothing new for people who sunk hours into playing/fighting with their pals in NSMB Wii, but it becomes readily apparent early on in that adventure that destructive play is just as agonizing for yourself as it is to the people you're trying to cause pain to. Nintendo has given you a way out of this tormenting loop.
Four players can still jump around with uneasy enjoyment using the Wii Remote flipped on its side, just like they could in NSMB Wii, and there's little difference between the anything-goes craziness that happens when four players try to achieve the same goal in unison. However, there is one sizable change that brings virtual harassment to a new level. A fifth player can use the GamePad to join in the fun. No, this doesn't let you control a previously hidden-away fifth Mario brother (Peabo?). Rather, you assume the role of an all-powerful god.
Boost mode, as Nintendo calls it, allows the player holding the GamePad to place blocks in the environment just by tapping the screen. If you're in a helpful mood, you could use these blocks to aid struggling players so they can cross dangerous pits or reach dizzying heights without exerting much effort. This is a great way to introduce inexperienced players to the often challenging world of Mario platforming, so there is a definite place in Nintendo's ever-expanding audience for such a feature. But for those who were weaned on Mario Bros. and don't require such coddling, the ability to drop blocks wherever you please opens new avenues of schadenfreude.
As Mario, Luigi, and two different-colored Toads run around onscreen, you change the obstacles they face in real time with a tap of your finger. You can pick one player to torment if you choose, ensure a wall of blocks appear in front of Mario at inopportune places in unexpected moments, and generally be the nuisance you've always wanted to be. Or you can spread your terror around the screen. Maybe put a block in front of Luigi one moment and then freak out Blue Toad with a brick the next.
In addition to placing blocks, you can stun and then juggle enemies by tapping on them. Like the bricks, this is a feature that is intended to help struggling players, but it only adds to the chaos for a well-meaning griefer. Mario is going to find it a lot harder to maintain his balance on a rotating wheel when a koopa troopa keeps flashing in front of his path.
The ability to interact with the world while perched in a safe place makes for some explosively entertaining situations. Remember the old Bugs Bunny cartoon called Duck Amuck? In it, Daffy Duck is tormented by an unseen artist who keeps messing with the drawing without any repercussions. This is the video game equivalent of that classic cartoon. But just like trying to play competitively in NSMB Wii required discretion or else you would receive an abrupt game over, so too do you need a delicate touch playing the grief master in NSMBU. You don't want to kill your friends, after all, because that would end your fun as well. So you push them just far enough to annoy them without finishing them off, and then give a helping hand when they're just about to quit in frustration. It's a delicate balance.
The most difficult part of NSMBU is keeping friendships in the midst of the unrelenting grief you're causing. Just know that when the stage ends, you have to switch with another player, and you can expect to get the brunt of the brick-based warfare if you were a particularly nasty jerk. But all is fair in love and war. Whether you try to help or hinder your friends, New Super Mario Bros. U is a lot of fun, but it's hard to deny the potential for this new style of tormenting your buddies.