Bowser's plot to take over the Sprixie Kingdom. If the Green Sprixie Princess hadn't managed to go through the crystal pipe, thereby alerting the heroes to the danger, then it's quite likely that Bowser could have actually succeeded!
Rosalina is an unlockable character, and her ability is the spin attack from Super Mario Galaxy.
On the flip side, this means that Bowser and his troops—even Goombas or Koopas—can kill Rosalina.
The Golden Express, a golden space train loaded with coins.
Double Cherries, a concept that was created by accident, are used in several boss fights including the final one.
The final boss, which involves Bowser using a Super Bell to transform into a Tiger version of himself named "Meowser", then you climbing a tower while dodging his attempts to clamber around and destroy your footholds. Then, after you use a POW to land a hit on him, he uses Double Cherries to have multiple copies of himself continue the assault!
Let's put this another way if you can't grasp the significance: Bowser is using powerups. After all these years, he's finally thought to use Mario's powers against him. You thought the Cat form let you break the game? Guess what? The final boss just got ahold of that, and multiplied it by eight or so for good measure.
Just to paint a picture about the scene: The climax of the final showdown has you reaching the top of Bowser's skyscraper while being increasingly swarmed by several Clone Meowsers that are climbing on all sides of the building as you reach the goal, while another Meowser roars on top raining fire down from above while the others attack from the side. Not to mention the intensifying music adding to the final moments.
The music alone is enough to deem the whole battle as awesome.
In one of Captain Toad's stages, he finally gets to kill an enemy by falling onto it from above. In the last stage, after being harassed by a Fire Piranha Plant throughout the entire level, it ends with Captain Toad killing it the same way. Captain Toad is far from the Miles Gloriosus he was in the Galaxy games, to the point where he received his own game, Captain Toad: Treasure Tracker.