For me, a lot of the things I enjoyed about the Pokemon franchise went away in the transition to 3D. The Pokemon games where consistently some of the best games on their consoles before Gen 6. When they tried to make 3D models for everything, the game lost its visual identity for me.
Back when handheld gaming hardware was more primitive, Game Freak were the kings of cramming as much depth as possible into tiny ROM sizes. Seriously, think back to Gen 1. As broken as those games were, they undoubtedly had the most depth of any other games in the original Game Boy's library. They managed to fit an entire adventure in 500 KILObytes. Those games were legitimate technological achievements.
I also found the sprite artwork on the older games to be a lot more compelling. By the time Gen 3 rolled around, it really felt like Game Freak knew what they were doing when representing their characters on a small screen. The simplicity of the medium allowed them to get the point across much more effectively. Now, Pokemon games look like a shitty Pixar rip-off. Half of the 'mons look like bizarrely smooth jelly beans with arms. Its just not practical to make over 1000 compelling 3d character models, complete with battle animations. How the hell are the creatives at the top supposed to audit that much content?
I feel like a lot of the skills and artistry that went into making premium games for 32-bit hardware are becoming lost. Powerleveling a bit, but I program for absolutely dogshit hardware comparable to the GBA a lot in my day job. As an adult, its given me a deep respect for Game Freak accomplished 20-30 years ago.