Thought I'd post some later-game comments in case anyone was still curious...
I just completed the initial 6 worlds to the game. I've enjoyed it very much and have been challenged without being completely overwhelmed... for the most part. If we're talking about simply completing the levels and progressing, the challenges and increase of difficulty goes along at a perfect pace IMO. If we're talking about completing levels well enough to earn the gold stars necessary to unlock the expert levels, that's a whole other ball game...
For the 1st world, I earned stars often on my first run through each level with little difficulty. World 2, took a few repeats but no real trouble. World 3 and on... ugh. The Mini-Mario and Donkey Kong levels for each world were the exception, I earned most of those without much effort. But for the average levels it was often all I could do to get even half the score needed to get a star. I'm not sure I even know how
to do better. If I try to get the present boxes that takes more time... less score. If I skip the present boxes that means I miss out on the few thousand points they represent... often enough to rob me of the star. Seems you have to be quick and
get the presents too.
I don't mean to sound too
critical... I like the game a lot and enjoy a challenge, it's just that the difficulty in that regard ramps up a lot after the 1st two worlds. But at the same time, the Mini-Mario and Donkey Kong levels at the end of each world, which I would assume would be the hardest, were often a cakewalk.
After the initial 6 worlds (plus a challenging level tacked on to the end... a very nice homage to Donkey Kong IMO) I now have access to the 6 "plus" worlds and the "expert" levels. The plus worlds are an interesting twist on the original gameplay... instead of each level being broken down into 2 parts, you must find a Mini-Mario (who's holding the key) and lead him to the exit. There is no Mini-Mario level, but you face a Donkey Kong level at the end. Much like the initial world, I blew through Plus World 1, winning stars on each easily. The Donkey Kong level was pretty much just like the 1st, but much harder. It was the first DK level I thought was challenging.
The number of expert levels available depends on how many stars you've earned. At a quick glance, I've got a dozen open to me now. I just fired up the 1st one for a few moments and it looked much more complex than anything I've seen yet.
All told, it's looking like there's a lot more to go... although I hope there's a cheat to unlock all the expert stuff as I doubt I'll ever get stars on all
of the initial and plus levels to see them all.
*EDIT* - Turns out I misread the expert level menu. You're shown a total of 12 expert levels, but right now most of those are grayed out. I only have enough stars to try the first 3 right now. So um... I've got a long way to go