The physics are slower. Think this is probably to put more emphasis on combat instead of running and hiding. There are some gigantic levels that may actually be bigger than Hyrule Castle (which is in Brawl as well) such as a complete remake of the original Donkey Kong's level 3 and a very weird level called New Pork City to name a few. Pikachu is very likely a top tier character when the 'official' list is released. Also to note the dodging in Brawl is much better than in melee; being able to dodge immediately backwards after a jump is a nice improvement as well as being able to perform multiple dodges in midair and still attack. The only thing that kinda bothers me is you can't dodge into the ground at an angle and slide, but hey only a few people actually used that technique a lot.
Like I said, I only played a few levels. Though I should have clarified in my post, I don't really like the levels that move. Hyrule Castle was the largest stationary level, and I didn't see one that large in the game for the small time I played it. It really is a group game and I haven't had the opportunity to explore the game well yet.