It's like Mario's entire platforming history got put in a blender.
http://www.nintendoworldreport.com/impressions/27820
Super Mario 3D Land is shaping up to be a great addition to the long-running Mario series. The game combines bits and pieces of many previous games, both 2D and 3D, while taking advantage of the 3DS's stereoscopic capabilities. Nintendo gave me the chance to play through the first world of the game.
The game combines some of the best aspects of the 2D and 3D series. For instance, Fire Flowers act as real power-ups as in the 2D games, but unlike those found in the Galaxy titles. Super Mario 3D Land takes many cues from Super Mario Bros. 3, including the return of the leaf power-up. The soundtrack also includes remixes from that game as well as sound-alike tracks, similar to those in New Super Mario Bros. The original Super Mario Bros. also lends a heavy influence, such as the return of the end-of-stage flagpoles and even coin heaven, yet Mario now has all his tricks from the 3D games.

One new item is a set of gyroscope-controlled binoculars that let you look deep into stages to help you plan for what's to come, as well as providing a different perspective on the level. Toad even rewarded me for exploring the level, dropping off extra coins after I zoomed in on him. Managing perspective and the stereoscopic effect isn't strictly necessary for clearing levels, but they're pretty important for uncovering medals and other hidden items.
There are also enemies old and new. While the staple Goombas and Koopas return, there are modified enemies like Tanoombas and Piranha Plants that spit black ink onto your screen. There are also new enemies, such as insects and tiny dragons that reminded me of Tamagon from Devil World. The first Bowser has a Tanooki tail -- but you can't grab it; it's spiked. In a throw-back to the original SMB, you must defeat Bowser by casually walking by him and knocking out his bridge. When you do, "Bowser" reveals himself as an imposter Goomba with a leaf. It seems many enemies are now capable of gaining Tanooki powers.

Perhaps to alleviate some of the problems with control on the 3DS, levels in Super Mario 3D Land, or at least the early levels that I played, feature relatively linear segments. There are plenty of secrets to be found by straying a bit off the path and checking every nook and cranny -- with many hidden by perspective and optical illusions. Control works reasonably well on these straight portions, but becomes more difficult when floating through 3D space with the Tanooki suit. The Circle Pad is simply too imprecise compared to a regular control stick.
Levels primarily go in one direction, but that isn't limited from left to right or even back to front. One level I played was directed downwards in some of the most striking stereoscopic 3D visuals I've seen on the system. The guy playing next to me was actually getting vertigo from it. The effect definitely does make dropping down more dramatic.
Miyamoto has been talking about trying to bring in those apprehensive with the 3D games. In my experience, new players have an easier time exploring an open space than trying to keep steady down narrow paths. Since levels in Super Mario 3D Land switch orientations and directions frequently, I don't think Nintendo is going to be very successful here, even with the 3d-but-nearly-2D design.
With the game now nearing completion, the Warp Zone in World 1-2 (which we discovered in the E3 demo) now leads directly to World 2. The first world was completely linear and contained four main stages, Bowser's castle, and a Mushroom House. Stages are unlocked by collecting medals. Sadly, Nintendo wouldn't let us take a peek inside the house. The map was laid out similarly to Super Mario Galaxy 2, with 3D representations of the levels placed in a line.
It really feels like Nintendo's having a lot of fun designing this game, taking what's worked well in the past and seeing how many new ideas they can combine. The game takes heavy cues from the 2D games, while using 3D effectively. While I was apprehensive after the first time I played it, now I can't wait to see more.