I have a large amounts of complaints about XY.
When I saw Lysandre I immediately said, "That guy will be the leader of Team Flare." That's how painfully obvious it. Same for Diantha for being the champion. I also found that they tried to make her too similar to Cynthia. Overall the plot and characters were a major step back from Black and White and was overall a huge disappointment for me. The only thing that made me like X and Y were a few of the new Pokémon (but some still sucked), and the new features (which are made completely obsolete by ORAS). Team Flare is also the worst evil team in the series especially compared to Team Plasma and the Gym Leaders were so bland and boring. The Battle Mansion is the most boring Battle Factility, especially right after a game with the PWT. Kalos was also very boring, and all it really had going for it was the French theme. The locations were boring as well. Santalune Forest is literally a clone of the Viridian Forest, from the location to having pikachu, while in DP there was the Eterna Forest which had the Old Chateau and BW had the Pinwheel Forest which had a really cool design. And don't get me started on the Haunted House from Kalos. I was extremely dissapointed with that. So disappointed that I didn't give him a tip (I usually will because I think it raises the chance of finding a shiny). All the caves were very bland and forgettable. Most of cities (with the exception of Lumiose) all seemed the same, and Lumiose City is hell to navigate across. And I thought Castelia City was hard to travel through. Also, XY has the least amount of new Pokémon, including Megas, while BW introduced 150, one of from Kanto. And for the most part, they were good designs, although some did suck (Vanilluxe, Garbodor), but every region has its fair share of bad designs. A lot of people will say that at BW they were starting to lose ideas, but if anything they are now due to the low amount of new Pokémon.
TLDR: X and Y have a massive amount of flaws