to sidestep Oedo and lolmonade duking it out here, I'm going to quickly put down the list of changes I think Sakurai probably SHOULD have mentioned during Nintendo Direct rather than what he actually did.
* All aerial attacks have had landing lag almost or entirely removed. paired with the new ability to hit A+jump to perform short-hopped attacks, this means a hell of a lot more diversity in attack options.
* Air Dodge Lag, especially when it's a directional air dodge or an air dodge after you've built up the new dodge fatigue a lot, is practically a death sentence to use off-stage. air dodge was such a good option in smash 4 that it made stocks draw out and proper off-stage battles nigh impossible because anyone recovering worth their salt could easily air dodge and get back to the platform no problem.
*The speed of knockback has been increased
*the effectiveness of Smash DI (the treehouse used the term DI, so it's cannon) has been decreased, but regular DI seems to be more effective
*throws knockback farther across the board, making them worse for combos and better for kill moves.
* Overall speed increase, as mentioned in above comments.
* Dash can now be canceled into grounded normals. this means you're not committed to running attack, jump, shield, or the occasional up-smash from running. the Direct mentions running Smash Attacks, but the same is true of running tilts as well. again, this along with the lagless short-hopped normals makes for a VERY diverse and offense-charged neutral
* STAGE HAZZARD TOGGLE! GameXplain was BEGGING For this one. in addition to stages having their omega and battlefield forms, you can also turn off most of what a stage does and have what competitive smashers wanted the whole time, which is stages that have the occasional moving platform to give SOME terrain to navigate during a battle,b ut nothing overtly crazy random about it. Granted, this won't make all stages able to be unbanned because who wants to have a 1-on-1 in Great Cave offensive or a stage where a character with a wall jump can camp out a character without one all day like Saffron City? there's also stages with ridiculously small blast zone spacing like the Game n' Watch stages too, and I don't think it's reasonable to have a blast zone standardization option.