It's great for the exact same reasons that every 3D Mario game is great: top-notch gameplay and top-notch level design.
And on top of that, in the eye candy department it's stunning. The reviewer that said that after playing this game he/she realised that everyone else had been doing HD wrong was entirely correct. It can easily stand next to the best that the PS4 and One can offer without looking out of place and completely obliterates anything that the PS3 and 360 are capable of.
Like others have said, the first 3 worlds are for n00bs and the game really comes alive from the 4thwworld onwards.