Awesome Bosses: Pretty much every boss from Soda Jungle on. That includes Bowser Jr. and the minibosses.
Awesome: Video Game Levels: Superstar Road-2: Run For It! It's an incredibly fun challenge of racing the P-switches' time limit over coins turned into brick blocks. There's very little room for error and you'll likely suffer several deaths on a few first tries, but you'll have a blast doing it.
Of course, to others, it qualifies as That One Level instead because of the necessity for constant running, the precision timing needed for the last couple of switches, and having to memorize the level to avoid death.
Breather Boss: Boss Sumo Bro. As long as you keep hitting the platforms he's on until you're able to hit him, you probably won't take any damage at all. You can even knock him down immediately after he recovers.
Breather Level: Superstar Road-9/Follow That Shell!, while still tricky in its own right, is a breath of fresh air to players, especially if they had just finished Pendulum Castle.
The same could be said of its New Super Luigi U counterpart, Flying Squirrel Ovation.
Broken Base: With the announcement of New Super Luigi U, there were cries from some of the base that feared having to pay another 60 dollars for NSLU on top of the 60 dollars for the main game. Nevermind the fact the price of New Super Luigi U was unannounced around the time of the broken base, and that the base game easily has more content than any New Super Mario Bros game to date. Fortunately fears were eased in the end. The New Super Luigi U DLC only costs $20 bucks to download and for collectors, a retail version of NSLU exists, priced at $30.
Ear Worm: For some, the main theme will never get out people's heads.
Even Better Sequel: Accusations of sameness aside, it's generally considered to be better than its Wii predecessor, if not the best of the whole subseries, especially because it manages to be better designed for 4-player co-op, yet is more challenging in single player.
Game Breaker: The P-Acorn seems to scream this, since it's the Flying Squirrel power-up but with unlimited air boosting. Its usefulness takes a hit in Co-op, though, since the four players will inevitably get in each others' way.
Bubble Baby Yoshis can turn just about every enemy into bubbles, even Boos, filled with coins or even power-ups, and said bubbles can even be used as a temporary bouncing platform. This feature, combined with the floating ability of the flying squirrel power-up, can make it very easy to perform speed-runs.
Surprisingly, the Super Star powerup has its place. It's invaluable for quickly catching Nabbit in most levels, as the speed boost and invincibility make it must faster to capture him.
Nabbit in the New Super Luigi U mode could also become this. Despite not being able to use items, he takes no damage from colliding with enemies, even at one point colliding with a King Bill and still not taking damage!
Bony Beetles, which are a cross between a Dry Bones and a Buzzy Beetle. Not so bad, right? But, like before, every few seconds, they'll pull in their heads and pop out some spikes, temporarily turning them into The Spiny. And when they reassemble? They do so with spikes, stopping you from stomping them immediately. When you're trying to maneuver around them, it's likely that you'll be hit a few times.
Sledge Bros. While they're only one one level, said level features moving platforms that you can be crushed between. What do they do? Create a minor earthquake when they jump, stunning you temporarily and putting you in danger of being squashed by said platforms.
None of the secret exits are easy to find on your own, especially since most of them are hidden in secret areas behind seemingly-solid walls. The same goes for some of the Star Coins.
It's not exactly intuitive that the rubber platforms in Screwtop Tower can be used like blocks under enemies, especially if the ripple caused by jumping on one doesn't hit the enemy above.
It's Easy, so It Sucks: The reception to almost every boss, especially the final boss. Any Koopaling can be easily defeated if the player just chases after its shell and immediately jumps on it, immediately crippling its pattern.
Also Unstable Equilibrium: If you don't jump on the Koopaling as they recover or before they begin fighting, they become more of a challenge to hit and avoid. Wendy almost invokes this, as her attack forces you to wait a second before jumping, as her damaging rings are created above her head.
Most Annoying Sound: If you fail too many times in a particular level, Luigi will put a box close to wherever you're starting from which you can hit to get an indication of how to beat it; this is accompanied by a doorbell sound, which plays constantly to remind you of its existence. If you fail in the same level repeatedly, this will rapidly get extremely irritating; not least since the doorbell is nothing more than a reminder that, so far, you suck at this.
Older Than They Think: Surely Mario was the first character who wore a flying squirrel suit to glide, right? Right? Nope, in Tomba! 2: The Evil Swine Return, released in 1999, the title character got to wear one.
Surprise Difficulty: If you've played the other games, the fairly high difficulty contrasting the cute themes probably won't surprise you anymore. The challenges, on the other hand, take things to a new level.
That One Boss: Iggy; while not really hard, his random pattern can get quite a few players off, what with his tendency to come out of the same pipe he came in, or when he shoots two fireballs that summon Magmarrghs, potentially trapping the players.
Roy fights using a two sided Bullet Bill cannon. And some of the Bullet Bills home in. Not to mention how he keeps jumping on the moving platforms...
Wendy. She goes against the player's instinct to go for the head immediately, as she summons her damaging rings above her head, forcing you to wait to hit her and possibly risk damage.
Kamek, since the only way to hit him is with one of his summoned enemies or to navigate the crystal blocks he creates to try to jump on him in time. When he disperses the blocks, they hurt you if you're underneath any of them, and they cover most of the screen. His magic blasts turn the solid blocks you're standing on into temporary Donut Lifts, forcing you to keep moving, too. The level preceding isn't easy, either, so have fun!
Swaying Ghost House due to how puzzling it is, especially its secret exit.
Red-Hot Elevator Ride. The premise is that you're on an elevator in a tower filling with lava, and you can tilt the controller to move the elevator. Goombas, Bob-ombs, Bowser Amps and para-coins fill the level. The only problem? The elevator shuts down if anything other than Mario lands on it. That means you'll be preoccupied getting rid of Goombas, Bob-ombs, and even coins so you can keep moving through the level.