Apparently, the "XY to ORAS" changes are here -
https://bulbapedia.bulbagarden.net/wiki/Generation_VI#Alterations_from_Pok.C3.A9mon_X_and_Y
Off the top of my head:
Pokémon Trainer customization, for sure! Not sure about any other major omitted features..although Pokémon Trainer PR videos, natural objects, and Sky Battles should all have been retained. I dislike limiting "features" to a region/set of games unless absolutely necessary.. Particularly with customization..non-French people play dress up too, you know, Game Freak.. -.- Additionally, I don't buy the excuse of "letting Brendan and May retain their original/unique/distinctive looks from Generation 3", as arguably their outfits have been altered noticeably/substantially in Generation 6, and they even don entirely different outfits for Pokémon Contests (-Spectaculars)! Hopefully, the feature returns in full force for Pokémon Z (if it will exist) and Generation 7 and beyond. I mean, if Game Freak wants regions to have their own "identity" apart from other regions, then the regions can do that with the regional geography and (most) of the Pokemon Trainers that the region in question contains (after all, each region can't contain the
exact same people and places - primarily due to plotline and variety purposes).
Oh, and the XY Berry mechanics should have been retained! (if they were good, that is) Also, I think there was an issue with the ORAS Berry Trees having a discrepancy with different Berry coloration and/or Berry number/count in contrast with the XY Berry Trees...? Something about the unique Berry Tree sprites or something about Berry Tree not accurately reflecting the Berry number..? ~_~
The XY Friend Safari mechanic should have also returned, although in a non-online dependent manner (or at least a manner where you can still reap all of the benefits if you don't have other players to connect to). And a Hoenn-style Battle Chateau should have returned from XY as well!