Greyhawk
What hooked me?/Lost me
I was never especially hooked. Some of it is fun and entertaining but a lot of it is rife with some of the D&Disms I find most irritating. The naming conventions are truly awful. I can forgive mundane names, especially if they're meant to signal that the residents of such a place are fairly typical sorts. The anagrams and high level of goofiness (I count militant neutrality as extremely goofy) put me way off.
Dragonlance
What hooked me?
Dragons of Summer Flame. I read it before I encountered any of the other books. I liked the Age of Mortals a lot. The notion of a world actually ruled by gigantic, cannibalistic dragons appeals to me still and the quasi-medievalisms are good fun.
What lost me?
The Saga rules just did nothing for me. I also became less and less enchanted with the desperately serious romantic side of the setting. Reading the original trilogy was another nail in the coffin. And tinker gnomes.
What might get me back?
I sometimes putter away on a much, much less romantic version of the setting that has a bit of a dark medieval flare. It's not really Dragonlance anymore, but it uses some of the elements. But in my version, the Knights of Solamnia are usually a bunch of lawful neutral jerks that like to lord it over the peasants whilst simultaneously bleeding them dry. Also, the Test of High Sorcery tends to brainwash those who go through it. You must foreswear all gods to enter the Orders, which are political and arcane cabals instead of alignment-driven. And druids are animists that the "good" gods and "evil" gods are keen on eradicating. In general, I abandoned alignment-based setting design entirely in favor of a set of mutually-exclusive and mutually-hostile worldviews. They're all fairly intolerant of one another (and detect evil will show anyone that's not sharing your worldview as "evil") and each is meant to have good sides and bad sides to it. It's a very gray place that likely would alienate the entire core audience for DL product. But I'm not selling it.
The Realms.
What got me?
I like highly-detailed high-fantasy worlds with scads and scads of magic and history and gods coming out their ears. A lot. It's my preferred setting. High-level D&D is comfortably superheroic for me, so I have no problem with high-level NPCs or demigods running around. Players do not ask me why Elminster isn't fixing everything for the same reason they don't ask me in a superhero game why Captain America or Superman or Other Powerful Superperson isn't fixing everything. Nor do I mind if any or all of these guys have wild sex lives. I got into the Realms after the heavy period of real-world pilfering, so things like Maztica, Kara-Tur, and such were beside the point.
What lost me on arrival?
Everything I've heard about the 4e Realms to date. Not one piece of good news in the lot of it.
What might get me back?
I'd like to say that a big post from Wyatt, Baker, Ed, etc telling us that the last page or so of the Grand History of the Realms was a bit of a joke that went over very poorly and was ill-considered would do it. But to be honest I think that would still leave me pretty angry.
Eberron
What got me?
I don't think I was ever rightly gotten. I did like that the setting embraced a more early modern period vibe instead of quasi-medievalism. Thus the printing press, the train, and the warforged were features. Most of the villain groups are very cool.
What lost me?
I don't really like pulp or noir all that much in my gaming. They aren't all that evocative for me.
What might get me back?
I don't think any setting change would do it. I'd just have to want to run a game set in a circa 1650 (with select bits of 1840 and 1914) to be into it. I actually like Sarlona a fair bit more than most of Khorvaire. Aerenal and Q'barra are good fun, though. My one planned Eberron game was going to extensively involve a Q'barra modeled heavily on colonial Africa, right down to racist Galifaran expatriots cursing the locals as lazy good-for-nothings unfit to govern themselves.