I know lots of people have issues with Kalos as a region in general, but my issues with it mainly amount to two things.
1) The music - I wasn't a fan of most of it. By that, I mainly mean this. A lot of the GENERAL themes were great (vs Wild Pokemon and vs Trainer in particular). But the themes that were supposed to be really important and hype worthy like they were in other games fell flat (Lysandre theme, Team Flare theme, Gym Leader theme, Elite Four theme, hell, even Diantha's felt downgraded). with the exception of the Rival theme, which I loved. It was just hard to be hyped by most of those themes. Even the city themes...I only remember Lumiose and Anistar's.
That being said, any piece that dealt with AZ or Looker...that was good music. Those ones felt emotional, like they should have. It's just the ones that are supposed to be hyped up that fell flat with me. Also loved the Credits theme.
2) Going off that, my OTHER big issue was the fact that the Gym Leaders were awful compared to previous ones. They didn't help drive the plot at all. There's no excuse, since Gen IV and V did the whole "Gym Leaders doing things outside the Gym" thing really well. It felt like a huge step back.
But other than those two things I loved Kalos.