To me, the biggest five mistakes they made was destroying hitstun, L canceling, tripping, nerfing characters that weren't even good in melee, and adding unlimited air dodges. Honestly everything after those is just nitpicking, wavedashing was really of limited usefulness for a lot of characters and would be nearly useless in Brawl considering characters like Samus and Mewtwo who depended on it were gimped so horribly that nobody would play them anyways or flat out removed.
Tripping is just stupid, I've only died to it directly about three times in Brawl but the amount of ruined attempts at combos and damage I've taken from it is really unforgivable. I wish they'd take their heads out of their ***** and patch Brawl to remove tripping.