New Super Mario Bros. U
From playing a slight amount of New SMB U single-player, and a more substantial amount with two, I can really only describe it in terms of New Super Mario Bros. Wii -- specifically, that the denser level design caters more to the single player and small crowd than the party situation encouraged by the Wii version. I'm looking forward to playing more of the Wii U version as if it was a traditional Mario game.
At the same time, I haven't even tried the GamePad control mode yet, so there might be some party-mode surprises left in there.
Progress: Stuck in a ghost swamp!