An Ice Person: The Ice Flower returns from Super Mario Galaxy. Although it's less "turns you into ice", and more "Fire Mario, but throws Ice instead of Fire". Penguin Mario can do the same thing, but he can also slide (and has normal traction on ice).
And Your Reward Is Clothes: Inverted: Mario loses his hat if you have 99 lives (he gets his hat back should his lives be less than 99 by the end of a level). Nothing happens to Luigi or the Toads, though (even though Luigi does take his hat off briefly whenever he wins).
Anti-Frustration Feature: Failing a level at least eight times activates the "Super Guide"; selecting it will let you watch a CPU Luigi play through the level. The player can resume control at any time, or let Luigi finish the level and then choose to either try it themselves or advance to the next stage. It does come at the cost of 100% Completion, as if you cause the block to appear (by losing eight times) the stars you get on your save file won't twinkle.
Blatant Item Placement: Oddly enough for the franchise, Justified. The Toads who stayed at the castle aided Mario, Luigi, Yvan, and Wolley on their mission by shooting power-ups throughout the land out of a cannon, scattering them across the lands.
Boring, but Practical: Shaking the controller in midair makes your character spin and keeps them in the air slightly longer. It's not a spectacular difference by any means, but it's absolutely vital to controlling your jumps and making those tough landings. (The same action triggers propeller flight when Mario has the Propeller Suit.)
Boss Remix: The music for the initial battle with Bowser is a heavily arranged version of the main overworld theme.
Convection Schmonvection: Everything ELSE about lava and volcanoes is portrayed somewhat realistically in this game, down to wrinkly, rippling air from rising heat and World 8-1 going so far as to sic a rapidly-advancing ash cloud on you, but Mario still doesn't get hurt from lava unless he directly touches it.
Difficulty Spike: The first four worlds (mushroom, desert, snow, and beach) are, for the most part, perfectly manageable, with possible exception made for World 3 until you get the hang of sliding/keeping the Penguin suit. But the last four worlds (jungle, mountains, clouds, and volcano), not so much.
Disc One Final Dungeon: The Airship levels. After each castle, it becomes clear that Peach is aboard Bowser Jr.'s ship, but you never find her on the ship levels. After beating Bowser Jr. for the third and final time, Peach gets taken away again just as Mario and/or the others reach her.
Disc One Nuke: The Propeller power-up is incredibly useful for many of the later levels, including the final boss — and it can be acquired easily in level 1-1!
Easy-Mode Mockery: If the Super Guide was made available to you for any level (even if you don't actually use it), the stars you earn on your save file after beating the game won't sparkle. This was done so the Super Guide could be both a tool to assist newer players, and a goal to avoid for the better ones. And if you use it to the end of a level, it skips the level for you, which means it doesn't count as being completed. Which means using it on castles and towers, which are the most common places to use it, will result in the victory flag not being there afterwards.
Faceship: The airship that the Koopalings and Bowser Jr. ride in has Bowser's face at the front.
Floating in a Bubble: One level in World 7 has water bubbles you can swim in. This is also a game mechanic in multiplayer: pressing A locks you in a bubble, allowing someone else to do a tough part of the level. If every player does it at once, you're all booted to the map screen.
That's only the start — the modding community for this game is surprisingly large and active.
Geo Effects: Just like in Yoshi's Island, Kamek swoops in during each Koopaling rematch to sway the odds in their favor by enchanting the battlefield somehow. He also does this during his own boss fight.
Giant Mook: Nearly every classic enemy can be found supersized. Some even have multiple levels of it.
Gimmick Level: A curious example, as nearly every stage features one or two level-specific enemies or gimmicks.
Griefer: Admit it, you picked up one of your teammates and threw him into a pit or into lava or purple slime, didn't you? And between players stealing power ups from each other, bumping players into enemies, and so on, many players will spend more time trolling each other than making any actual progression.
Guide Dang It: In order to unlock the world 9 levels, one must find all three Star Coins in every level in an earlier world. These coins can be very hard to find or acquire without help. Most of them do have subtle hints that mark them out from the rest of the level, but how to get them is another trick entirely.
Levels Take Flight: Two examples: One in World 5 involves Mario navigating across a flock of large flying manta rays going in his direction, and one in World 7 has Mario crossing a swarm of flying beetles smaller than the rays (but still very large for bugs) going in the opposite direction from him.
Literally Shattered Lives: Averted. Though you can freeze enemies in ice blocks and then shatter them, if you look closely, only the ice shatters, while the enemy falls offscreen in the usual fashion.
Lost Forever: The sparkling save file stars, if you ever make the Super Guide box appear on any level at any time.
Mercy Mode: Failing a level at least eight times lets you use the "Super Guide" feature, where a CPU Luigi runs through the level you're having trouble with. However, having the Super Guide appear on your save file removes the sparkles on your save file stars.
Mickey Mousing: Inverted. Some of the music is apparently so catchy that some enemies and even some power-ups and Yoshis (when players are not riding) will dance and sing along at certain cues.
Mini-Dungeon: As usual, the mid-world Fortress levels, as well as the Ghost Houses.
Mythology Gag: As with the DS game, there is a homage to World 1-1 from the first Super Mario Bros. and a Bowser battle that can be defeated simply by breaking the bridge and dropping Bowser in the lava (or down a pit ala Super Mario Bros. 3), replacing the axe with a button.
Never Say "Die": The instruction manual refers only to "blunders" and "making mistakes" while still saying you "lose a life."
Press X to Die: In multiplayer, you can press A to enter a bubble, just like the one you end up in after dying in the level. Guess what happens if you press A while all the other players are in bubbles? However, no lives are lost directly — every player just gets thrown into small form and kicked out of the level.
Recurring Boss: Bowser Jr. in the latest model Clown Copter. Each Koopaling is also fought twice, with a different attack pattern.
Recurring Boss Template: When facing any of the Koopalings in the tower stages halfway through each world, every battle is a relatively straightforward affair of dodging their wand attack and then delivering a Goomba Stomp to the head (much like they were in Super Mario Bros. 3, for that matter). This is not the case when facing them again, in their castles at the end of each world.
Ring Out Boss: Inverted. Lemmy is incapable of actually harming you (unless you jump into him like a moron), and instead tries to knock you into one of the bottomless pits on either side of the arena with his bouncy rubber balls.
Schmuck Bait: Yes, don't mind the cake with 6 different hair styles, a bow, and sunglasses on it....
Secret Level: World 9. Some other levels are borderline — in Worlds 4 and 6, there are only eight levels displayed in the Star Coin menu initially, but when you clear the castle of those worlds, a ninth level is suddenly revealed. Likewise, only nine levels are depicted in World 8's Star Menu until you clear the Airship level and a tenth level is revealed with much fanfare. World 7-6 and 8-7 are kinda-sorta examples in that they're already on the Star Coin menu to begin with, but they are obscured on the map until you find the secret exits in World 7-Tower and 8-2 respectively.
Skippable Boss: Although all bosses except Iggy and Bowser Jr. can be skipped before Bowser via the Warp Cannons, the first battles with Wendy O. Koopa and Ludwig von Koopa can even be skipped within their residing levels. It turns out there's a secret exit to each of the relevant levels, the former leading to a cannon to World 6. The latter leads to a secret level that leads to a backdoor into Ludwig's castle, skipping most of the level and depositing you right at the door to the second fight with him, inverting the trope.
Smashing Hallway Traps of Doom: More accurately Smashing Tower Trap of Doom. 6-Tower (Morton's) has some incredibly Freudian spiked pillars that repeatedly jam into each other. (Previous levels have similar large black spiked... protuberances, too.)
Spin Attack: Spinning in the air gives you a tiny little bit more airtime (unless you have the propeller or are holding a propeller block/someone who does have the propeller, in which case you'll get a LOT more airtime and altitude.)
Spread Shot: Ludwig uses an Initial Burst in the second fight against him.
Stalactite Spite: Lots of icicles in the third world. In one level, the entire ceiling is full of them.
The Stinger: The Koopalings helping Bowser up from his shell... And getting their castle dropped on them.
Villain Exit Stage Left: There is a sequence after every castle where the character(s) run(s) up to Bowser Jr., who has the airship behind him, complete with Princess Peach on deck. Bowser Jr. taunts you and boards the airship, and you can do nothing but watch it fly away, except in Worlds 4 and 6, wherein Bowser Jr. gets left behind and your character(s) chase(s) him back into the airship.
Visual Pun: Most levels in the first world have rolling hills, and a couple of levels take it literally.