Mario Kart Wii took away co-op racing and snaking, and added 12 player online, bikes, wheelies, challenges, stunts, more characters, new items, new courses and older courses, more vehicles, more characters, Miis, and more control schemes. Most of that is just "new stuff," but the new drifting, new drafting, bikes-vs-karts, and motion steering all make the game feel really different.
Brawl added a level editor, online level sharing, online photo sharing, online fighting, more characters, non-Nintendo characters, more dynamic crazy levels, assist trophies, stickers, challenges, a co-op adventure mode, a co-op events mode, a co-op boss rush mode, more music than any game has ever had, more customizability, and more control schemes. And much to the dismay of the Melee tournament crowd, they replaced wave-dashing with random tripping, which is awesome. And they nerfed Fox. Ask any fan which one is their favorite and you'll get a really long speech. These games are crazy different.
There's a reason Smash and Kart sequels don't go 1, 2, 3, while games like Halo and God of War do. Those are direct sequels that are continuing the same story, they can't change the game too much without ruining it. They need to improve some things here and there, but keep the game the same enough to finish the story. Kart and Smash are always re-invented to utilize a new console's features and you only get one per console.
I'm not bashing Halo or God of War here. I'm explaining why direct sequels don't change as much as certain Nintendo franchises. Look at Star Fox. It still can't decide what it wants to be.












