Well it's easy till the end of World 7 I guess? Starting from World 8 all the way to Special 1 to Special 8 it's a good mix of easy and hard levels IMO.
And I always turn on my 3D no matter the game (damn you Devil Survivor Overclocked), but the 3D effects from SM3DL takes the cake. It's just amazing, not gimmicky at all. That's probably the standard Ninty wants to make. 3D for a purpose.
Plus it's a perfect mix of 2D and 3D elements of a Mario game.
Playing this game tempted me to go grab a new Wii and play SM Galaxy I & II, but well I'm cash strapped after I spent too much this year. (Half of my 5-year-old arsenal of games were bought this year)
That being said, SM3DL packs a "Wow" factor, but there still seems to be something missing. It lacks the "WOW" factor. It's full of O.O moments but it still lacks a proper jaw-dropping moment.
But it's still a pretty darn fine game. Well at least I feel that the wait for this after being an early 3DS adopter was worth it.
NOW GET ME ANOTHER POKEMON GAME FOR THE 3DS.