I hated the confusing damage multipliers. what, with types and such. that said, I played Gen II first, then Gen III, then, and only Gen I. It just didn't make sense to me. It incorporated ONE type and I often didn't know which.
I hated Zubats and Geodudes and Tentacools coming at you bro every five steps.
I hated Brock with a passion. Pikachu just isn't meant to do that sort of thing, and I didn't like Mankeys.
Diglett's Cave. Oh, my God. Fu-... uh, Fork that stupid cave.
>enter with Pikachu
>level 30 Dugtrio
>challengeaccepted.jpg uploaded
>few moves later
>Pikachu fainted!
>kills whole rest of party
That said, I didn't like having to start with perhaps the weakest pokemon in the game.
Grindan' in that game -- most difficult task ever. It is simply the worst. It took me the better part of half an hour to get Pidgeotto to 36 from 30 in the mid-20's grass outside of that one city, the one with Koga... I forget its name.
SILPH FORKIN' CO. Hated that God-forsaken place, with its oh-my-God-I-want-to-commit-murder teleporting panels.
All in all, it's a fine game, and it was the start to the best series ever made, but it wasn't my favorite out of said series. That was Silver, with no close second. Far second, sure, Black is awesome. But that's getting off topic.