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.
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.
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.
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.
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.
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.
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: With most bosses, this can be avoided, as you can stunlock them, but a few bosses can become this if you fight them the normal way.
Wendy, if only for summoning her rings above her head to prevent you from charging in to jump on her. Once you get past that, she's not too bad.
Iggy isn't too hard, but he's rather difficult to stunlock, and his unpredictable pattern will get some people (Sometimes he pops out of the same pipe he just went into)). Not to mention, if he summons a Magmaargh on either side of you, you're pretty screwed.
Roy, for wielding a double-sided Bullet Bill cannon. The moving platforms don't do much to help.
Superstar Road-2, to others, 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.
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.