I just completed the game 100%!!! And I did it all without looking up a walkthrough! The only hint I got the entire way through was Daroga's "It's not what you do, but how you do it," hint, which was vague enough that it still took me awhile to figure out what he meant, so unlocking world 4 was still very rewarding.
If you want to know what you get for completing the game 100% read on.
(spoilers follow).
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... not much. You get a message saying "All spent" and a second star next to your name in the menu. And the blue toad house in world one has 4 bonus lower screen wallpapers you can buy. That's it. Of course, if you're playing a Mario game for its extras you're in the wrong place, as the fun comes from playing the game and discovering the secrets on your own.
(End spoilers)
And here are my 2 cents on the game:
I liked it (of course), though in some ways I was dissapointed. While I didn't keep track, I'd say it only took no more than 10 hours to unlock everything. And I found the mushroom houses essentially useless (I never used one once), since lives and power-ups were so plentiful. Those were my main complaints.
As far as power-ups, the blue shell kind of sucks, but no one's forcing you to use it, and you'll only need it on rare occasion to unlock secrets. The mega mushroom was cool, and I liked that it was used sparingly. It was a fun gimmick, but nothing more. Ultimately, those 2 power-ups added little to the game, but they didn't detract at all. But the real show-stopper was the poison mushroom, which you'll use a lot for getting to secret areas, and I liked that while it makes you the most agile, it also makes you the most vulnerable adding some challenge to the game if you're trying to unlock everything. If there's one thing I'd like NSMB to add to the Mario cannon, the poison mushroom would be it.
But aside from my minor quibbles, the game was great, with some of the finest level design ever in a platformer. Compared to the other mario games I'd say Mario 3 had a harder difficulty, but fewer secrets, where as Mario World was real easy, but had lots of secrets. NSMB had all the easiness and secrets of World, but had the shorter levels, smaller scale, and more simplistic feel of Mario 3. Overall I'd say NSMB was perhaps a hair below those earlier games, but still a worthy sequel nonetheless.