I remember how excited I was for the new 100 Pokemon coming out in gen II. Then, when Gold and Silver came out it was perfect. It still managed to include all of the Pokemon from the first generation (aside from legendaries, starters and fossils), while focusing more on the newer ones.
Then Ruby and Sapphire came along with another set of Pokemon, and this is what pushed it over the edge for me. I still put a lot of time into this generation, but it didn't feel like the same series. It just sort of felt like they were milking the success of the first 2 generations, further proven with the releases of FireRed and LeafGreen.
I played through Gen IV and V too, but hardly played them after finishing the main story. There is just too much now, to the point where it's just overwhelming. Not only are the designs of the pokemon worse, but it's impossible to complete a pokedex without trading... a lot...
Maybe it's because I've grown up, but I know if I was a kid getting into the series right now, it'd just be way too much for me. Getting into the series and trying to collect all 151 was amazing whereas collecting all 700 is just overwhelming. It's a shame they couldn't find ways to keep the series fresh other than adding more pokemon.