My wife and I finished world 8 in New Super Mario Bros Wii the other day. We felt very accomplished; the final levels were as difficult as anything in 2D Mario. Having a wife who plays Mario is great. I love you sweetie!
Even my wife commented that the final showdown with Bowser felt more like something out of Sonic than a Mario boss (see the end of my Mario Galaxy 2 thoughts). Not that I mind. My favorite Mario games feature great bosses to complement the great platforming, like Mario World 2 and the recent games.