Well, they really wanted to take the nostalgia here literally by mainly sticking to the original Hoenn Dex (and as a result, most of the Gym Leader/Elite Four teams retained the same Pokemon), and as for trainer customization it was revealed that the feature was mainly to showcase the "beauty" theme of the Kalos region so that's why it was an XY exclusive feature.
That doesn't make it a good excuse though! I mean, look at HGSS. They did the opposite to FRLG with the nostalgia factor, which the latter did by not even allowing newer evolutions to happen until postgame (including Golbat --> Crobat), a decision that is not well supported by the fandom. HGSS kept the same general spread of Pokemon obtainable in routes ala GSC, but they didn't stop people from say getting Tangrowth, and greatly expanded the number of Pokemon obtainable before the E4 through both the PokeWalker and Safari zone. And this I feel worked very well - for anyone who wanted to stick to the old pokes, they could very easily ignore the Safari Zone and Pokewalker Pokemon! If they didn't want a Tangrowth for whatever reason, just not let it evolve! And for those who wanted or enjoyed more options, it's all there.
ORAS did not do this until after the 8th gym, which is a bit late imo. The same goes for the lack of trainer customisation. 'Nostalgia' is a bad argument, because if people wanted to keep playing as May/Brendan, they could have those be the default models, and they can just ignore the stores selling clothes! For everyone else, there'd be the option.
I don't really like the fact that you need X/Y/AS/OR to complete the national dex
That's always been a feature of Pokemon however, and encourages the buying of games and moreso the trading of Pokemon aspect Pokemon has been built upon.
Stuff I didn't like...
- still didn't like the rivals! Wally was better I suppose, but Brendan/May and that forced shipping by the game during the Delta episode... no thanks. I'd have rathered they had a personality that defined their character.
- no berry mixing game wah
- Trick house why are you so much more boring and lesser in the quantity of challenges
- Battle Frontierrrrr
- 3 pokemon limit in secret bases
- those rock smash obstacles north of (New) Mauville still
- removing the 'explore to find Steven' part of Granite Cave.
- Forced teleportation by Wally after 5th Gym battle
I'm sure there's more, but that'll do for now. =p