3D World is a strange Mario game, mostly cause it attempts to do basically every single thing a Mario game can do.
You have 3D platforming mechanics, like the long jump, triple jump, side jump, back-flip, and more. You have 2D Mario game mechanics, like life being tied not to a meter, but rather the amount of power-ups acquired. And, of course, the flagpole.
And yet even with all of these elements going on, there's no clear sense of direction or identity. Super Mario 64 has the excuse of being the first 3D Mario game, and it's identity is really built via it's tone and worlds. Super Mario Sunshine has FLUUD and the tropically themed Isle Delfino.Super Mario Galaxy has the spin attack, gravity, and the beautiful cosmos. 3D World has no consistent world identity, and has so many different game-play mechanics that trying to look for a central concept there is a fools errand. Maybe it's the cat suit? Maybe it's the clear pipes? Maybe it's the 4 player co-op itself? Those nice world maps? It's hard to know. Despite all this, 3D World is an absolutely brilliant Mario game, featuring many of the best levels and overall moments that the franchise has ever seen. Bowser's car boss fights make for such a great format for multiplayer co-op boss fights. The overall use of sports objects as projectiles is really fun. Baseballs, soccer balls, many of them are in the game... for whatever reason. Sure, I'd put a few other Mario titles above it in terms of quality, even it's predecessor, Super Mario 3D Land on 3DS, which I believe does the more isometric level design much better with the 3D effect that the 3DS produces. but it's still a fantastic video game.And that goes for both single player, and co-op play. You can't really go wrong with 3D World.