Through the courtesy of baby Yoshis, we can finally kill boos by eating them!
On previous fights with Bowser, all you do is press some sort of button that brings down an enormous Bowser. In this game, you actually get to face off with him, and bring him down by smashing him on the head three times with the Koopa Clown Car. With awesome graphics!
This needs more elaboration. You are fighting Giant Bowser head-on, for the first time in the series. It's not like Galaxy 2, where you shoot meteors at him, no—you are right up to him, in a twisted Peach's Castle with a dark hurricane swirling in the sky and the glow of fires below. You rob Junior of the Koopa Clown Car to hit Bowser, but Bowser does not just let you do it, he has two different anti-air attacks to keep you away. And when you do score a hit? He recedes into his shell and chases you, going no slower than your top running speed. he then uncurls, followed by raining fire from the sky. There's also a bit of Fridge Brilliance, here—not only do you fight Giant Bowser directly for the first time, but the method to defeat the Koopa King is essentially stomping on his head.
Bowser deciding to just toss away the Mario Bros. instead of letting them chase after him is both savvy, and awesome of him.
The entire "Boarding the Airship" level, especially with the clouds and sunlight in the background, dodging the giant claw throughout, and ending with a fight against Bowser Jr. and tricking him into crashing the airship himself.
You actually have the option to send Bowser a letter via Miiverse if you die too many times on a course.