Just copying my initial impression from somewhere else.
Mario 3D World is one of the best Mario games I've ever played. Like Galaxy 2 it's just filled with idea after idea. I had a chance to play 3D Land before I played this and the controls are MUCH tighter. Mario doesn't feel nearly as floaty and doesn't lose as much momentum as he did in 3D Land, making precision platforming a lot easier. Each of the characters really feels different too. I almost always switch in between Peach and Toad just because of the floating and speed boost. I can't tell you how many times Peach's floating has saved my life and Toad's speed actually made me go TOO fast haha. Most of the moves from 3D Land return like the charged crouch jump, long jump, roll, rolling long jump, and side jump. There's also a new move where you spin around for a second and then do a spin jump that does a high jump that makes you floaty as well. Great for getting up higher than normal ledges or trying to get the top of that flagpole haha.
The new power-ups and weapons are pretty sweet. The cat suit is probably the most useful suit in the game. Being able to climb walls is so much fun, even when there's nothing to actually find. Using your claws to attack on the ground is so useful. And being able to jump off a cliff but climb back up has saved me numerous amounts of times. Other items that return from 3D Land actually aren't all that prevalent. I've yet to really see the Propeller Box (which, in all honesty, would be horrible in multiplayer) and while I see the Tanooki suit on occasions, it's mostly Cat Suits and Fire Flowers. Haven't seen the Boomerang suit yet. The Double Cherry is so much fun to use in levels, I hope to see more of it in future levels.
It's definitely a love letter to all Mario fans. Every level is littered with reference after reference. Captain Toad levels as a reference to Galaxy, little sprites of Mario and Luigi that appear in random locations (as in this picture, try and find the Luigi!), Mystery House levels that are much more challenging as a reference to Super Mario 3D Land, musical references to EVERYTHING, and so on and so on.
Speaking of the music, oh man. Probably some of the best of music I've heard from a Mario game. From the Jazzy grass levels to the violin-filled levels to the fantastic water levels to the unique boss music, it's just too much to talk about! I can't wait to get this soundtrack the second it goes out. I neeeed iiiiiit!
This is definitely a game I can't wait to play multiplayer with. It's just so smooth! I highly recommend it!
Now that that's out of the way, yes, I picked it up. It's pretty awesome to say the least. As of that post I hadn't played multiplayer but the multiplayer is a whole nother experience. Whether you're trying to do it cooperatively or just messing with your friends, it's so much fun. Definitely a step up for New Super Mario Bros. U. No offense to that game, I liked it, but it doesn't hold a candle to this one.
I've already beaten it so feel free to ask me anything else.