These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: New Super Mario Bros. U
Best Boss Ever: Pretty much every boss from Soda Jungle on. That includes Bowser Jr. and the minibosses.
Best Level Ever: 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 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.
Character Rerailment/Divergent Character Evolution: An arguable case. The Koopalings have been given a bit more personality and different weapons. Lemmy has a circus theme going on in his airship, Wendy's has heart shaped windows and she ice skates across the arena, Roy has a double ended Bill Blaster which he uses like it's an exercise gadget...
Ear Worm: For some, the main theme will never get out people's heads.
Even Better Sequel: Accusations of sameyness aside, it's generally considered to be better than its Wii predecessor, 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.
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!
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, they become more of a challenge to hit and avoid.
Of course, some might argue that this case doesn't apply to the Boss Sumo Bro.
It's the Same, so It Sucks: Despite it shaping up to have its own identity, this game gets called out for not bringing out "the full potential of the Wii U".
Many fans are upset that the Yellow and Blue Toad are returning as players 3 and 4 instead of, say, Wario and Waluigi. Again.
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...
That One Level: Swaying Ghost House due to how puzzling it is, especially its secret exit.
Snake Block Tower and Red-Hot Elevator Ride can be frustrating as well.
In Superstar Road, all the levels are difficult, but the three hardest levels by far are Run For It, Swim for Your Life!, and Pendulum Castle.
And for the Challenges: Don't. Touch. Anything. and Screwtop It Up.