The hook is definitely the aesthetics, which were famously so over-the-top they made Nintendo backpedal towards safer portrayals of the Mario cast, but the gameplay is also really solid. It's chaotic but still requires strategy, there's good variety between the different characters, the item system is a great way to encourage careful play, and it's just satisfying to charge up and score a Megastrike. And this is a minor detail, but I love how this game uses motion controls. The fact you have to shake the controller to attack adds an element of brutality I don't think could be recaptured with buttons, like you're actually shoving your opponent in real life.