New Super Mario Bros. Wii
I didn't get as much face-time with New SMB Wii as I wanted - because it was pretty much the star of the show, even at NOA's mini-E3 employee event. Comparisons are being drawn to Four Swords Adventures, and those are pretty apt, but there are two big differences.
The first one is obvious: this is a platform game. Where an adventure like Zelda can get by with new dungeon puzzles and enemy encounters that get mixed up by having more heroes, a Mario game lives or dies purely by its level design. (Although now that I think about it, multiplayer boss battles sound like a blast.) The good news is that based on the stage select screen in the demo, you can expect a ton of level variety from this game, from fields and deserts to haunted mansions and oversized clock interiors.
The second difference, and I could be off about this, is that while FSA filled in your missing players with dummy Links, there's no such thing going on here. I can't imagine how there could be - since keeping four dudes in "formation" in a Mario game would basically be impossible, as would be moving one at a time and switching control between them.
This is an important point because it means the single-player game will be fundamentally different than the multiplayer one in how it plays; and possibly in how challenging the game can be made. The people I watched playing New SMB Wii weren't exactly great at it, but it still looked to lean toward the tough side, as if it was assuming that not everyone would make it to the end of the level.
So will the game be no fun in single-player? Will the level of challenge be capped, making certain levels trivial with four people? Or maybe - and this is almost purely speculation - there could be some sort of auto-tuning difficulty...?