One thing. Latias. That alone makes me love Hoenn.
Why, just last week I finally completed my Hoenn Dex.
There are many kinds of things in Hoenn. The first game with a full fledged Ocean, not the "rivers" in Kanto and Johto. (They were oceans, but you were forced to go a certain direction.) Also, the Battle Frontier makes me want to keep on playing. I'm still trying to develop a good competitive team, and eventually port it over to take on Sinnoh's Battle Tower (and Platinum's Frontier, but that's another story).
Not to mention TV, sandstorms, new bikes, all that junk. But I can't miss it because I didn't get rid of it when I got Diamond.
I also play Emerald more frequently than LeafGreen for some reason...