I find it rather odd that Wet-Dry World is a later stage. There's no bottomless pits in the stage and because of the water it's easy to refill your health. I guess I can see Nintendo feeling the village part that requires you to be good at wall-jumping should be later, but even then, some of the earlier levels have stars I'd considering harder as well.
At the same time I've always found Cool, Cool Mountain one of the harder levels to get all the Stars in and yet that's one of the first in the game. It's pretty easy to slide off the mountain if you're not careful, as well as the penguin race can be pretty tricky the first few times. I honestly feel that's a harder snow related level then Snowman's Land, despite that level being a later one as well.
Of course part of me wonders if this was intentional. Since you only need 70 stars to just beat the game, they put some of the harder levels earlier so it wouldn't be a complete cakewalk. Since 99% of the stars can be taken in any order, if the game had a more linear difficulty, players could get almost all 70 stars in the easy levels by the time they reach the third floor. That would certainly explain why something like Shifting Sand Land is earlier in the game when it's right up there with Tick Tock Clock and Rainbow Ride in difficulty.