I can't pick just one thing, so I'll mention a few (a lot work for both XY and ORAS, but whatever):
Horde battles: I am terrible at Super Training, so I'm glad XY also introduced these to make EV training easier/quicker. I've also caught several shiny Pokemon on both XY & ORAS from Hordes, so there's that too.
Mega Evolution: Probably my favourite battle change/mechanic/thing since the Physical/Special split in Gen 4. It's encouraged me to use Pokemon I haven't used much before and/or wouldn't put on a main team before, e.g. Mawile on my Y team.
Battle Maison: (Very) frustrating at times, but I find it to be a very easy and fast way to earn Battle Points (Y is the first game where I've ever maxed out my BP).
Other things: Destiny Knot IV breeding, Battle Chateau & Lumiose Restaurants, Pokemon Amie