I've always been a little sceptical of the direction Mario took when going 3D. People say Mario 64 was a perfect translation of Mario into a 3D space, but I tend to disagree. Mario 64 and it's sequels have always felt like a different type of game to me: more of an adventure game than a pure platforming one. Yoshi's Island brought some exploration to the 2D series before 64 came along, but even in that game platforming was always the focus.
Although I enjoy these more adventure-ish Mario games, I've always missed the "flying by the seat of your pants"-aspect of 2D Mario games. I'm delighted to say that 3D World, especially when playing as Toad, makes you play as fast and loose as you'd like. I've been devouring stages in rapid succession, reading the landscape as it presents itself and improvising as I go. A lot of 3D World levels do this even better than most 2D Mario levels I've played. It's fantastic.
A more critical note: the fact that 3D World places more importance on defeating enemies by jumping on them (because by default you don't have a "2D Attack" like the spin move in Galaxy or kick in 64) re-introduces some perspective problems that have plagued Mario since going 3D. I'm having a hard time jumping on enemies from certain viewpoints, and having a locked camera makes getting a feel for the space in three dimensions harder. This is exacerbated by the sometimes flat lighting, which have made me miss jumps due to not being able to see the depth of the scene properly. That's something other 3D Mario games have tackled more effectively in the past (Galaxy most succesfully IMO, by moving the spherical playing fields and de-emphasizing precision jumping in a 3D space).
All in all though, if I had to recommend a single Mario platformer to someone, I'd 100% recommend this. The amount of content is staggering, and the quality (some small annoyances aside) is through the roof.