Irying this demo, i found it quite interesting, although there are few things i don't really find appropriate (kinda the first submap: It looks bad! A lot of Back Door's towers to make the "shop center" palace, where you enter and there is flat NESish graphics with high detailed characters, but the rest is okay).
First, the levels: They are all ok, regrading level design, but with few compliants, which one is:
This part is too hard: you should lower the other side of the waterfall thing, because that part is particularly hard, compared to the rest of the levels.
Then, if you want the cape required to find secret exits, you might want to add at least one cape powerup in a level: no one is going to buy a powerup for 50 coins: it should at least be in one level of the world.
Some parts (the keyhole that leads to the yellow switch palace) are too hard to reach: lower that part a little.
Now, about the graphics you used:
Watch out the cutoffs here: you might want to add few tiles to fix the pipe-grass connection, the same for those pylon things.
The, the overall graphic choice: not bad, actually, but i think the palette contrast should be higher: they look too bright.
Then, i think the major compliant about the graphics choice is that all the levels look the same each other! The whole world seemed an incredibly long unique level divided by phases rather than individual levels, because you used the same graphics over and over: even Ice Man's Bowser'r Return did that, and that actually looked boring. You should vary the level graphics a little, no need to use the same graphics in all levels of a world.
Unless it's required, though.
then, about the boss:
Urgh no! This boss is the glitchy! And even if you jump on it, you might get hurt. This because of a spriter's laziness who didn't want to make a more reasonable boss (i mean, the author of that boss). I would change this with something of more appropriate, really.
Then, aside those compliants, i think this hack is on a good way.
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