Team Rocket were the biggest reason behind the OS' success. As I see it, them being older characters free of the stereotypical personalities of the protagonists were essential in bringing a wide variety of viewers to the show.
OS was the best representation of the Pokemon world. It was just the right blend of wacky, weird and fantasy, with an edge of danger that isn't present in other the other seasons. It's world is still ultimately a utopia, where nothing truly horrible ever happens, but it added wrinkles to that utopia.
Every gym leader in Kanto was unique and distinct personalities. Gym leaders became a lot less remarkable as the show went on, as they became upstanding people rather than normal people just doing a job who have their own circumstances. Does anything compare to:
Brock - Has countless siblings he has to look after because his parents abandoned them
Misty - Has a strained relationship with her sisters, creating a dysfunctional gym
Lt. Surge - A tough SOB who beat up people's Pokemon
Erika - Disliked people who disliked perfume, wouldn't even let them into her gym
Sabrina - Possessed by some spirit, turned Ash and friends into dolls and tried to crush them
Koga - A straight-up ninja
Blaine - A weird old-dude who fights in a volcano
Gym battles were fun because Ash always had to jump through hoops to even get one. That kid used a dubious machine on a stranger's advice to supercharge Pikachu, cross-dressed with the help of TR, and was crushed by a chandelier on the way to getting a ghost-type. In future series, all he had to do was turn-up.
I seem to have gone full genwunner here, but man, when I think about the AG-XY era, I find it difficult to distinguish each individual show because a lot of stuff just repeats itself. They change up the look, change the cast the bit, even change how each season feels, but they're still all structured the same way, they all re-use the same storylines, and the problems that existed in 2008 when I joined these forums remained problems up until 2016. Not that these seasons are bad at all, but it's telling to me that I didn't watch AG and DP while they were airing, and dropped BW and XY at about the 60 episode mark. Something's not clicking for me.