Recently I downloaded Super Mario Bros. 3 on the WiiU, and noticed some levels are quite harder than I remembered when playing about seven years ago. And some of the levels I've always declared the hardest are a little easier for me now (I've been better at exploiting certain secrets and stuff that are helpful). Of course some levels are mainly difficult if you start out as small Mario or can't figure out which power-up you should use when entering.
I wonder if there's a glitch when playing on the WiiU. I've noticed at times certain enemies don't appear at all, seems like it's if I have certain power-ups or something. I've looked at pages on the Mario Wiki and haven't seen any acknowledgement of these, so either they are glitches or secrets that have primarily gone undetected (but Super Mario Bros. 3 had its own strategy guide, surely it would have mentioned the possibility of certain enemies not appearing if you do certain things).
I kinda get why Goomba's Shoe only appears in one level (and disappears once you complete it), it might make things too easy, but couldn't they have put it in a few more levels, maybe one per world? In fact the one level with the shoe isn't really a hard level at all, with or without the shoe (and that's one of those glitches I mentioned - I decided to play through the level once without getting the shoe to make sure I was right about how easy it is, and didn't see the third Goomba Shoe).
The sun only appears in two levels. I can't really decide which one is scarier - his first appearance, where he's just standing still for the first half (though he does have that angry face) before flying out and trying to attack Mario, or in level 8-2, where he doesn't show up until halfway through the level, coming out from behind a hill (and if you are flying through the level, possibly off the hill, he suddenly shows up from off-screen). Actually, I've never been scared of the sun in his first level, but I can see how some might be scared. When I first played level 8-2 years ago, I was startled when the sun suddenly showed up (I think I had the p-wing or had run fast enough to be flying off the hill) - and I was aware that he was in the level. At least there's a secret passage way that allows you to avoid contact with the character.
But still, I think it was a missed opportunity that The Angry Sun does not appear in worlds 5 or 6, which would have been appropriate worlds for him (he could have been in the world 6 level where the ice had basically been melted - or maybe the level before that one).
A few weeks ago, I had a dream where I was using the secret passage way in level 8-2, and that the Sun followed me down the quicksand. Then I was playing that level, thinking about the dream, and a flying Goomba suddenly dropped down, not sure if this was a glitch or not (didn't really pay attention to whether he came from the ceiling or the quicksand), but with its shape and light coloring I thought it was the sun at first.
The white platforms that allow you to temporarily go behind scenery only appears in a few levels, but aside from being able to go behind and use scenery to protect yourself from enemies, it seems like the one that leads to the first warp whistle is the only one to really allow you to do/get something amazing.
Let me tell you from experience: The first fortress of world 7 allows you to get unlimited coins and extra lives (if you die from running out of time, you'll lose one of those lives, but can just stock up on more lives), but it makes things a little harder if you're plan is to keep dying so you can stock up on items from the mushroom houses for when you get to world 8 (or some of the worlds hardest levels, like 7-8 or the second castle). If you're trying to collect as many items you can and die on purpose so you can keep coming back and getting more, it can take a long time to run out of lives to go back to the refreshed mushroom houses again (even if the second fortress is hard enough that you're bound to lose all those extra lives unless you use the hammer suit).