I consider
1-1 in Super Mario 3D Land to be a work of art. (The link is to my thread on the subject)
1-1 in 3D Land was essentially a playground,
brimming with interactive bits and bobs, and PACKED with secrets. To me, it stands as one of the finest introductions to a game ever.
What's great about 3D World is I see multiple levels on display here echoing the design ethos of 3D Land's first level, with secrets packed into every nook and cranny, lots of unique traversal opportunities, and just loads of stuff to run over, leap on and off, etc. The clear tubes, the rolling wheels, the climbable blocks, the invisible blocks found with the GamePad, etc. It just looks exquisitely interactive, practically begging the player to come at it from every angle -- above and beyond the edge of the screen here, over this block and behind this wall there. I love it.
Also, I lost my shit nerding out over the return of Charging Chuck. Yes. Yes! YES!!