I thought I'd posted in this thread before, but I guess not. So I'll do so now! :)
All in all, there are a lot of factors new to ORAS that change things up significantly, even though it stays reasonably true to the original games. Honestly though, I think they've done very, very well with the re-introduction of Hoenn, and that it absolutely won't follow in the footsteps in the originals as 'the worst (or at least, the least successful) generation of games'.
Some of the things introduced, such as Mega Evolution and the renewed EXP share, definitely changed the way the game was played. There's no way around saying that the EXP Share made it substantially easier - I turned it off periodically throughout the game, and managed to be on a par with Steven by the end of the game, but it was hard to find a medium! When it was on, I'd become overlevelled before too long, and while it was off, I'd fall a bit short. It definitely taught me how to better (more enjoyably) utilise the new mechanics of the EXP share.
That said, there are some features omitted from ORAS that did leave me a little disappointed, such as the lack of an ability to customise your character. Plenty of people (even in this thread) have reasonably argued that the feature was present in XY because they were new games, but even so, I would've liked to at least be able to change the CLOTHES of my character, if not the hairstyle, etc. Although it did bring me back into that feeling I love from Pokémon games, setting up a canon, and whatnot.
I do agree with points 2 and 4 raised by Birthday Thunder above, though; people have argued the lack of a Battle Frontier with "It wasn't in the original games, so it won't be in the remakes.", and I honestly don't know what those people are thinking. To HELL with that! There are plenty of things that weren't in the originals - one of the major things wrong with FRLG was sticking with that sentiment, omitting features that could simply add to the renewed world. If ORAS left out Mega Evolution throughout the story, it would've been quite boring. But look what they did! They introduced it, and not only that, gave you access to a Lati mid-story. That was great!
I mean, the Battle Resort Island is nice and all, but the fact that it's still a Battle Maison sorta bugs me a bit. I would've liked it to be a Battle Tower again, although to be quite fair, I see why that's hard to reintroduce. The Maison made it super fast to blast through loads of battles, quitting whenever you want. A tower makes it harder, and would require more cutscenes and so on. Though even with all this in mind, I still would've loved to have played through a modernised Battle Frontier. It was a really fun post-game feature of Hoenn's, so it's a shame they left it out.
As well as that, the rematch feature of the Pokénav+ was greeeeeeeeat, but the fact that they plain just didn't let you rematch the gym leaders also sucked. Sure, another feature introduced in Emerald, but one that positively influenced the game, literally no reason to leave it out in ORAS (HGSS did it, even though it wasn't in the originals, so nobody start with that! :P).
Despite its few lackings, I really think ORAS pulled out all the stops, or at least most of them, and was a great playing experience. I've been tempted to get AS just so I can play the story mode a couple more times!