Well plenty has changed, even from just Gen V.
For one, the controls have changed quite a bit, with the new 8 direction circle pad. I personally loved roller skating in XY, and sneaking up to hidden Pokemon in ORAS is quite fun too. Also cool is the new soaring feature, which cuts out the need to have a HM slave for Fly once you have gotten the Eon Flute (you don't even need Latias/Latios to be in your team to use it!). Soaring is the only way to get to a large multitude of legendaries available in ORAS, so it is a must use feature.
As rarmstar mentioned, mega evolutions are a really cool new way of battling, both for competitive or just storyline play. The actual graphics of the evolution itself is pretty cool, and the occasional change in abilities/typing can really throw your opponent a curve ball. But personally I find Pokemon-Amie a far more appealing new feature. It allows you to personally engage with your Pokemon in some one-on-one time. For playing with it (petting, making faces, games) to feeding time, or even just prodding it awake when you first enter, it makes the entire experience much more real, making you bond with your team. This side feature also aids in gameplay, because the better your relationship, the higher chance of your Pokemon healing itself of status conditions (breaking through its paralysis so that you won't worry) or withstanding OHKOs (toughed it out to show its best side to you).
If you are into competitive battling, Super Training might also prove handy to inform you exactly how your Pokemon is getting along in the EV side of things. You can even train EV from the feature, though I told horde battling (also a new feature where you encounter five lower leveled wild Pokemon instead of just one) is much faster for EV training.
XY also offers customization which makes dressing up really fun (more so if you play as a girl) and gives your avatar more identity. Kalos is a very interesting region, highly steeped in style and culture so if you like just experiencing a new region, I highly recommend it. It has a wonderful atmosphere which makes just sitting on benches and listening to the soundtrack an enjoyment in itself.
And finally, the new Fairy typing is very fun to use, breathing life into a few old favourites (Clefable, Gardevoir, Togekiss) as well as introducing very cute but powerful new ones (Floette, Sylveon, Diancie). It really helps with the dragon typing imbalance, and nothing is cooler than seeing Klefki OHKO a Mega Charizard X. {XD}
