I wasn't around for the heyday of any setting but Eberron. I don't mean to insult others' personal preferences when it comes to setting, and I admit that I don't actually know a whole lot about Greyhawk or Dragonlance or what made them great during the TSR era. My understanding is that Dragonlance was popular largely because it had some really good fiction attached to it. I know this area is overwhelmingly subjective.
But I genuinely can't even describe how tedious I find Greyhawk, Forgotten Realms, and even Dragonlance. They are in my mind, a vacuous black hole of anything interesting or flavorful, an irredeemable morass of the most generic and boring fantasy tropes imaginable. Never even really having experienced it outside of Baldur's Gate, I was tired of the Realms long before the Spellplague. I find it less and less interesting with each new cataclysm. I have difficulty imagining why anyone would willingly read another Drizzt novel. Obviously, though, Wizards has done their market research on this one, and they still expect the Realms to be their best bet for most supported setting in Next. That makes me sad. I can do generic fantasy on my own. It's the crazy and weird fantasy I wouldn't mind reading about.
I'd much rather see a return of the unusual settings, no matter how weird or downright silly they are. I'm interested in Mystara (and its attendant sub-settings). I'm interested in Ravenloft. I would look at a full treatment of Planescape. I would love a new Spelljammer. I was on board for Dark Sun. Al-Qadim, Maztica, and Kara-Tur all interest me, though their ties to the Realms is unfortunate. Birthright and Council of Wyrms? Bring it on.
This is one of the things I love about the various settings for Savage Worlds. They're never bog-standard fantasy stuff. They all have an interesting twist. Magical pirate adventures. Dark fantasy with sky captains and airships and a bit of post-apocalyptic flavor. Fantasy in which eldrich horrors have already won and enslaved the whole world. A world that's a 100% traditional fantasy setting at its core, but it's overlayed with
pure undiluted insanity so thick you can only barely see through it.
But sadly, Wizards' desire to unite the fanbase has sadly led to the conclusion that the aggressively generic and boring must come first. Ah well.