The half-baked Kanto region was one of my biggest quarrels in G/S/C. Sure, we have R/B/Y, but way too much was taken out of Kanto:
Viridian Forest (in G/S/C)
Mt. Moon was greatly downsized.
No Legendary Birds
Cinnabar Island (probably the most famous omission)
The Gym Leaders were also ridiculously easy to beat, all the trainers were underleveled, and Kanto just felt, well, empty.
A great thing that they could've done was expand Kanto a bit, keep the original things in it, and add some "anime-exclusive" locations into it. I actually wish they would've done that with Kanto in general. Nonetheless, it's still my all-time favorite Pokemon region.