I mean designs overall, just look at how nice the first two generations look, the third one already borrows ideas from previous one(Tailow is pretty much a 3rd gen Pigeotto). It may also be the attachment to first two that play some role here, but IMO after 3rd gen, the designs look funky(especially Legendaries), good example would be of how the three starters Pokemon look in later gens, and how they don't even come close to first two gens.
While I agree that the split you speak of is usefull, this is like one thing good about next gens, and that's it. Pokemon X and Y introduced Mega Evolutions, and while they are cool, I just think it's kinda pushing it, a feature to even further make fanboys go wild and buy the game.
I watched some LP of Pokemon Y, and I have to admit it just keeps on getting worse, the further technology in gaming develops. To me it was charming as 2D, The 1st, 2nd and 3rd generation really were great, but with 4th and upwards, it just keeps getting worse. New Pokemon designs which make no sense, adding of new types...
I know it's off-topic here, but I just had to rant about it a bit