Best Level Ever: The Spine Coaster level (8-7) has become the favorite level of many fans. 9-8, the "cloudpoline", is also a lot of fun without being too easy and contains a nice holy crap moment near the end.
Breather Level: A few, including 7-5, 8-2, and 9-6. There's even a shortcut in 8-2 that leads to 8-7, considered the awesomest level ever. If you want to complete the game quickly, the 8-7 shortcut leads to the final airship.
Contested Sequel: Only natural, given that this is the first 2D Mario platformer on a console in nineteen years. It has been accused of being less creative than previous games.
Crowning Moment of Awesome: The final boss battle. The return of Giga Bowser, giant lava, and a somewhat challenging platforming section make this final boss memorable.
The comedic collapse of Bowser's castle during the ending sequence. Becoming fifty feet tall and going on a rampage turns out to be a bad idea when you're storming through the foundations of your own castle.
Notice how, after being beaten from their towers, most Koopalings make three hops to their castle. Now notice how Wendy makes two hops, Morton makes five hops, and Kamek makes no hops at all. This is all meant to foreshadow the fight with Bowser Jr. in those worlds.
There are certain places in the game where it's possible to farm an infinite amount of 1-UPs. The game even kindly shows you how to get them.
The Propeller Suit, continuingthe tradition of flying powers allowing you to breeze past levels. It isn't quite as broken as the cape from World since you can't skip levels entirely, but many levels, as well as getting the star coins in them, becomes significantly easier.
You can also knock out Bowser Jr. in the second airship battle with a Propeller Hat instead of the cup used to knock him into the wall, similar to the first battle with him.
The rescue bubble in multiplayer also qualifies. Normally, you respawn in a bubble when you lose a life, but pressing A does the same thing and lets you keep whatever powers you have. This makes falling into bottomless pits impossible and can make levels extremely easy if one player is blazing through and carries everyone else while they're in a bubble. Of course, if everyone goes into a bubble at the same time, it counts as a screw up.
In "Bowser's Inside Story," Bowser had his castle fall on top of him. The same thing happens at the end of this game's credits.
It's Easy, So It Sucks: Accused of this in single-player, since most of the platforms in the game are rather large instead of the trickier small platforms in other Mario games. This is probably justified in that the game was designed towards a multiplayer focus.
It's Hard, So It Sucks: Becomes this in multiplayer, as many feel that the level design isn't large enough and/or is too fast-paced to accomodate more than one player.
Scrappy Mechanic: One of the most common criticisms of the game was having to shake the Wii Remote to spin jump, get off Yoshi, and pick up objects, despite them leaving out the B button on the Wii remote. This could cause lags in actions that lead to untimely deaths.
"Seinfeld" Is Unfunny: This game was originally praised for being the first 2D Mario platformer on a home console since Super Mario World. Recently, however, the game has been criticized for leading to the stagnation of original ideas in the Mario series, with nearly every subsequent game in the series being influenced by it in some way.
Morton Koopa Jr. The first time, you have a small amount of space to avoid his shell. It's big (so you have to jump high) and fast (so you have to jump often). This is made worse by the constantly dropping spike pillars on the edges of the arena, more or less forcing you onto the small arena for the most part. Then there's the second battle with him where he can do a One-Hit Kill based on where you're standing in relation to him. If you're on a section of the floor adjacent to one he's on, get away.
Wendy's second battle can be quite frustrating. Not only is she fought in a room that periodically fills with water (making her unable to be Goomba Stomped), she fires these hard to avoid rings that bounce around the room and don't dissipate for a long time.
7-6. A scrolling level that involves you jumping from beetle to beetle to make it to the end. Depending on who you are, this might not be too difficult. But if you're also trying to get the coins, it can be somewhat torturous and having Yoshi doesn't always help. Also the fact that jumping from beetle to beetle yields extra lives and if you keep dying and running out of items, it will drive you crazy.