It probably says more about me than it does about the game, but I'm mid-way through the Mushroom world and I've just be demolishing everything that has come before (usually getting all the starts and the stamp in one run). The challenge is completely non-existent. I think I'm getting to the point where just completing a Mario course is completely trivial for me.
I miss the sense of place that the prior 3D Mario games had (even the Galaxy games). I no longer feel like I'm exploring a world. I liked that feeling because it made the experience of playing the game about more than just completing the course.
The sheer-creativity here is incredible, don't get me wrong. I suppose I'm just less interested in repeatedly showing that I can clear the obstacles (having done that in Mario games since I was born) and more interested in just moving through a space that I feel a part of. For example, seeing a distant ledge and wondering if I'll ever be able to reach it. 3D World rarely offers that feeling. When it does, it's usually fleeting. I will have reached that hidden ledge moments after realising it's there.
Minor quibbles. Extraordinarily creative, but maybe the first signs of EAD Tokyo-style Mario fatigue, on my part. It's becoming a little bit routine.