GameplayNSMB:MSA isn't super hard, but it does provide a fair amount of challenge. The controls are the same as the original, with L and R finally making an appearance as a trigger for the Spin Attack.
World 1-1: A simple romp through grassland, stomping goombas on the way.
World 1-2: A slightly harder level than World 1-1.
World 1-3: More grassland.
World 1-4: A tower where Bowser Jr. awaits.
World 1-5: A walk through a damp area, with a few puddles.
World 1-6: More grassland.
World 1-7: Need I even say it?
World 1-8: Finally, some diversity! Prepare for a Bowser fight.
World 2-1: TROPICALITY. (It's tropical.)
World 2-2: Bouncin' on some pretty tall palm trees.
World 2-3: Slightly less tropical and a lot more water.
World 2-4: Bowser Jr. again.
World 2-5: Heck. I don't even need to tell you. More tropicality.
World 2-6: Well, I'll be darned. Tropical islands.
World 2-7: More tropicality, with darker water.
World 2-8: BowserCastle.
World 3-1: DESSERT! Oh, it's desert. Aww.
World 3-2: ENEMIES IN BLOCKS!
World 3-3: More heat.
World 3-4: Hot Tower
Worlds 3-5 to 3-7: Yeah, I'm doing the _______ to ___________ now. Lotsa heat, with an evil sun in 3-6.
World 3-8: Bowser once again.
Worlds 4-1 to 4-3: COLDNESS.
World 4-4: The Freezing Tower.
Worlds 4-5 to 4-7: More Deep Freeze.
World 4-8: Bowser is freezing. Put him out of his misery.
Worlds 5-1 to 5-3: Mushroom towers galore!
World 5-4: Bowser Jr., will you never cease?
Worlds 5-5 to 5-7: Movin' the mushrooms up, down, and all around.
World 5-8: BOWSER!!!!!!
Worlds 6-1 to 6-3: It's a jungle in here!
World 6-4: Cease and desist, Jr.
Worlds 6-5 to 6-7: Just more jungle, now featuring Dorrie the Dolphin in 6-5.
World 6-8: Curse you, Bowser.
Worlds 7-1 to 7-3: A lake! Swim for your lives!
World 7-4: Bowser Jr, you're EVIL! (Well, duh.)
Worlds 7-5 to 7-7: More fish! EATEATEATEATEATEAT!!!
World 7-8: You should know.
Worlds 8-1 to 8-3: LAVA!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1
World 8-4: I think you know.
Worlds 8-5 to 8-7: Volcano.
World 8-8: The final showdown. Yes, it's Bowser. Why do you ask?
After watching a cutscene, you win.