Okay, here's the thing, as was "the thing" with SMG, BiS, and a few other games: The difficulty of the levels themselves? The organized chaos of multiplayer is probably a factor, but I actually die constantly. However, there's a million Item/1UP/Star Houses, repetitive/condescending instructions, and those parts where the enemies attack you on the map and you're literally forced to get three Mushrooms. The "difficulty" in most recent Mario games is the equivalent of a drill seargant who follows you around the whole time happily offering Gatorade and encouraging you to rest for a little while.
Also, the Yoshis do not have enough of a presence to justify being on the cover of the game. Also also, I think they started putting Mini Mushrooms in the game, then realized they didn't want to and forgot to get rid of the ones they'd already put in.