Actually, I like parts of Gen III - Coliseum, Gale of Darkness and FR/LG were ballin'. I just couldn't stand Hoenn. Before you play the nostalgia card on me, I'll say that I could care less about Johto and that Platinum is by far my favorite game in the series. Anyway, Hoenn:
-The water, doods, the water. I wouldn't mind this so much if they dropped the encounter rate, put in better Pokémon to find or better yet, reduced the amount of water.
-There was something off about the region itself, as well. Despite the amount of environments, it felt so...bland.
-I couldn't stand the style. I'm not talking about the Pokémon (like every region, I liked some and disliked others), rather, I'm talking about the sprite style itself. Compared to FR/LG, I think it looks ugly and bleah.
-The so-called rivals. 'Nuff said. Some people might be annoyed with Barry, but at least he had a fully-evolved starter!
I think there's more to it than that, but I'm not willing to play through the region again just to see what I don't like about it. I respect it in that it introduced a lot of competitive battling elements and new gameplay, but I still don't like Hoenn. I seriously had to force myself to play the game so I could Pal Park the Pokémon I wanted.
However, I still play through the other games in the generation - I just recently replayed Coliseum.
But looking at this weird and amusing little trend, next gen everyone will be clamoring for the good old yonder days of Gen IV. When Gen II came out, everyone cried for Kanto; when Gen III came out, everyone adored Gen II; when Gen IV came out, everyone stood up for Hoenn, etc.