mega evolved pokemon get their altered speed taken into account when deciding turn order immediately, instead of the turn after. so mega sableye can’t get prankster buffed on turn one while still retaining magic bounce, mega banette can get prankster buffed on turn one, mega beedrill doesn’t have to use protect to ensure it survives to turn 2, mega garchomp slows down immediately instead of the turn after, and so on.
z-moves are also introduced, they temporarily turn a regular move into a super powerful one. most z-moves have a power around 160-190 or so, and have no negative effects even in the original move did. similar to mega evolution, z-moves can only be used once per battle and require a special hold item to use. unlike mega evolution, every pokemon can use z-moves, and every move can be turned into a z-move.
also Kartana is introduced