There are so many things to choose! I think I'll boil it down to my top five:
5. Legends Galore
Having not played many of the previous Pokemon games (I joined in around Gen V), this was a great feature that allowed newcomers to catch many new Pokemon from games they may have not played beforehand. This was mainly accompanied by...
4. The Soaring Feature
What a great way to show off what the 3DS is really capable of. The Soaring mechanic, once unlocked, was a brilliant way to take your HM Slave for Fly off of your team. It was also accompanied by some GREAT music, but more on that in a bit.
3. The Elite Four Rematches
When I took on the League in X and Y once again, I was disappointed to find that they had made no changes to their teams whatsoever, and that their levels were exactly the same as they were before. Hell, they didn't even have any Mega Evolutions! Aside from Madam Champion, of course.
So, once I saw that the rematch of the League was available, I was hoping that, at the very least, they would have their teams adjusted and leveled up. And I was not disappointed in the slightest. Their teams are, as I expected, in the 70s region AND they have Mega Evolution's!
I mean, I got utterly curve-stomped my first time through due to how SEVERELY underleveled I was, but that just goes to show you how great they were.
2. The Music
And the trumpets they blow...
The music from the original Ruby and Sapphire, as well as Emerald, is some of the greatest in Pokemon history (next to the songs from Gen II and V, of course), with tracks like the Elite Four theme and Slateport City standing out. So, once the new games came out, the music was GLORIOUSLY remixed with a full orchestra of music, with tracks like the aforementioned given a new life. New tracks are also brought in for a breath of fresh air, and they do not disappoint (Wally's and Zinnia's Theme's especially).
And the Number One thing in ORAS is...
1. The Delta Episode
This. ****ing. Addition.
This addition is probably one of the best things in Pokemon Remake history. Granted, that's not saying much, but it's still great none the less.
The new lore, the new music, the addition of Mega Rayquaza, Zinnia...Just everything about it is so great.