The music: There are few gorgeous city/route themes in the game, but overall the soundtrack was quite bad, especially when compared to the soundtracks of the two previous gens. The "rock-ish" battle themes were awful, IMO.
The characters: For the sake of making this post shorter, I will compare the characters only with Gen V and IV characters. First, the rivals: We had three amazing rivals in Generation V and the three of them (well, at least Bianca and Cheren) wen through a lot of character development, plus their teams were at least decent. Barry was a pretty cool rival in Generation IV, not the best one, but far from the worse. He was memorable, funny and battling him was fun, specially in the post-game. In XY, though? We had Shauna, who is very cute, but sucks at essentially everything (battling included), Calem who not only looks bad, but is extremely weak and boring and also Tierno and Trevor... I don't even know what to say about those two. The worse part is that they go through ZERO character development, from the start to the end of the game, they are basically the same shitty characters.
Now, the champion: Diantha is so weak. Her team is so boring and doesn't fit her at all (besides Gardevoir and perhaps Gourgeist). Giving her two dragons in the Generation that Fairy-type was introduced (meaning 99% of the players would have a Fairy-type in their teams) was so dumb, plus Aurorus is just too weak to be in a champion's team. But the worse thing to me, again, is that she isn't memorable at all and do go through character development. Sure, Cynthia did essentially nothing in DPPt, but she still is the hardest champion in the series, having a well balanced team with perfect IVs. Alder was weak too, but he appears frequently in BW and B2W2, giving advice, he was a very present and memorable figure in Generation V (not to mention the thing about his dead Pokémon and he had great music themes). Iris was nice too, for a number of reasons.
Now, the "Evil Team": I remember the pre-release days, when a lot of people speculated about what role the "Team Flare scientists" would have in the game. There was a lot of expectations for Team Flare and its members, since the previous two teams were really great, but in the end, Team Flare managed to be even more boring and disappointing than Teams Aqua and Magma from Generation III. The sad thing is that I really like Team Flare's goal and I see a lot of potential on it, plus the Team Flare characters look so good, they are the best-looking team so far, without a doubt. It a damn shame Game Freak wasted the opportunity of making the best evil team in the series. Hopefully they will improve those characters in the next game.
The Elite Four was also quite boring and very weak and that is a shame because their designs are really good, especially Malva's. Why they didn't use Mega Evolutions and teams of six Pokémon is beyond me. In short: The Gym Leaders weren't that great either, sadly. The only truly memorable character in the game was the Sycamore, since he battles the player and he is also hot. In terms of characters, Generation VI is easily the worst one so far.
Besides all of I've said, there was a lack of stuff to do between the third and eight gyms, you essentially only walk from a city to the other, beat the weak gym leaders and move to the next one, until the """"climax"""" of Team Flare's plot and then you go to the league and then comes the end of the game. Almost literally the end of the game, since there is almost no post-game content, besides the Looker/Emma side-quest.
So, I really don't understand when someone says Generation VI was awesome and say, Generation IV was bad. It just doesn't make sense. Yes, there is some stuff XY did right, but overall those two games are probably the worst ones in the series. Hopefully they will make TONS of improvements in the third version/sequels. If not, then I hope there are remakes to maybe redeem this generation. Somehow.