Here's a post explaining why this might be upsetting for some people (everyone who's saying "buy the new games to get the new stuff, duh, every other franchise is like that", read this):
XY and ORAS are both
6th generation mainline Pokémon games. A
new generation begins with a new game that introduces new Pokémon, a new region, changes in gameplay mechanics, etc.
Traditionally, Pokémon players have come to expect near-perfect compatibility between games in the same generation. You are able to trade and battle with no limitations between Ruby and LeafGreen, or Black and White 2, for example. Whenever there were new mid-generation additions to new games, they were handled as such:
Gen IV
- In Platinum, new forms for Rotom and Giratina were added. These forms were not in Diamond and Pearl (the previous games), so the solution Game Freak found was to revert these Pokémon to their original forms when connecting to previous games.
- In HeartGold and SoulSilver, new Poké Balls were introduced that didn't exist in Diamond/Pearl/Platinum. The solution was to prevent these items from being held by Pokémon, so they couldn't be traded. There was also a special Pichu form that couldn't be with you when connecting to other games.
Gen V
- There were new forms for some Pokémon introduced when going from Black/White to Black 2/White 2. These forms could be used in battle against BW players, who would see them as their normal forms (while they kept all the stats and attacks of the new forms).
In games that included hundreds of Pokémon and items, this was a very low number of compatibility "issues" that were always resolved in some (sometimes not very gracious) ways, but the compatibility was near ~100% between same-gen games. And even then, the DS didn't support game patches, so being outraged by the issues listed above would be even sillier.
ORAS are now taking a somewhat new approach of adding a very significant number of items, (Mega) Evolutions, attacks and abilities that just don't exist in XY's data (the latter two are a huge red flag for breaking compatibility). All of this while still being a Gen VI game.
With all these changes, the precedent of near-perfect compatibility between same-gen games, and the 3DS supporting game patches, it's not hard to see why some people are disappointed at the number of new additions they won't be able to share or use against XY players.