Broken Base: This is the first Nintendo game to feature paid Downloadable Content. Whether this is a sign of Nintendo's modernization or its descent into greedy corporate hell is still hotly debated.
Contested Sequel: The game's reuse of assets from New Super Mario Bros. Wii, such as the graphics, music, and engine, has been a major point of contention. Some fans don't mind and still think it's a solid game, where others have been angered and have accused Nintendo of getting less creative in its Mario titles. Still others fall somewhere in the middle.
The Invincibility Leaf. If you die five times in a level, this item shows up in the beginning of the stage. Like the name says, you can't die (except if you fall in a pit or in a lava pool) and you have a full P-Meter, so you can fly all over the level.
It's Easy, so It Sucks: One of the most heavily criticized aspects of this game was the lack of challenge. Getting to Bowser is pretty much a cakewalk; the game doesn't get noticeably difficult until you get to the bonus worlds, and even those are not as brutal as we've come to expect from this series.
Nightmare Fuel: There is nothing okay about the giant Boo (Boohemoth) that follows you through most of the Ghost House in World 2. It's huge, it has a terrifying face, it doesn't follow typical Boo rules (it will inch forward even if you look directly at it), and if you touch it? Instant death.
That One Level: A few of the warp cannon levels can be nightmares, especially World Flower-Cannon.