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.
Best Level Ever: The Bonecoaster 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.
Freud Was Right: Just try playing 6-Tower (or indeed, any of the levels with the huge black spiked pistons) without making a joke or two. The fact that the two pistons in 6-Tower jam against each other doesn't help.
Fridge Brilliance: 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.
Game Breaker: 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.
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.
It's Easy, so It Sucks: Accused of this in single-player, due to the fact that 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. However...
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.
That One Boss: 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.