Each new Mario game introduces new powers, and 3D World puts the new cat suit front and center. It's awfully adorable, but Nintendo has a knack for mixing in elements that are more substance than mere style. The cat suit feels specifically engineered to deal with this kind of space. Its climbing ability is well suited to the vertical space that litters most stages, while its diving pounce attack gives more options for dealing with enemies lurking on lower elevations.
The Double Cherry, which replicates your character, makes the action more frenetic and challenging. It's very difficult to keep all your Marios in a row, so to speak, so getting them all to a goal line with some prize for having three or four is a clever way of keeping stages challenging by demanding perfection. They can also be used to collect coins quickly, or produce multiple fires or boomerangs from those respective suits.
The pacing is also broken up by a few distinct stage types. One of my favorites, the Captain Toad stages, could almost be made into a small eShop release by itself. In it, you control both an explorer Toad with a heavy backpack, and the camera around him in a full cube-like 3D space. Since he can't jump like his heroic friends, these puzzle stages are more about moving the camera to see where he can walk or fall, and avoiding traps along the way. A handful of stages also have you rush to figure out and then complete specialized challenges in 10 seconds or less, invoking the frantic rush of a WarioWare game to gain more Green Stars.
Those Green Stars are scattered around ordinary stages too, giving Mario veterans more to do. Though the main quest to defeat Bowser and free the Sprixies doesn't get extremely challenging until the very end, tracking down Green Stars will often require much more precise skill and patience--and more than a handful of lives, in some cases.
Oh dear, Bowser has turned to kidnapping tiny ladies