Bowser got a little Genre Savvy; he is not vulnerable to attacks from Mario's powers anymore. If it weren't for Boss Arena Idiocy, you couldn't beat him.
The final fight is a running battle through the castle. Correction: You run, Bowser snipes at you with fire! In the second half of the battle, he gets pissed, and his flames turnblue-purple, and go screaming by with a sound like a jet!
Then he goes all Donkey Kong on you by trying to throw a barrel. The most tense part has to be when it all culminates in a mad dash to the end.
Peach fighting back against her captors, according to the photos you find in between worlds. Though she obviously doesn't succeed, it was rather nice to see, seeing as she often doesn't appear to put up any kind of resistance when she's captured, despite being more than capable of kicking ass in games where she's aplayablecharacter.