I threw my meager votes behind a few. Starting with my least hoped for and leading on:
Birthright. Had never seen a darn thing about it until I stumbled onto Harbinger of Doom's
blog entry about it. In reality, I'd probably never run a campaign in Birthright, but the
idea of it (every PC is superior/special) is enchanting.
Dragonlance. Never played in it, never read a novel set in it, but have tangentially touched Dragonlance ideas, writers, and ideas. Seems like a classy setting and I'd enjoy an excuse to get to know it better.
Dark Sun. Sounds like a blast; never played in it, but have read enough to know that I have a batch of players who'd be freaked out and loving it. I don't think they would have before, and I certainly wouldn't have, but I'm excited by the prospect.
Eberron. This is the crowning achievement of D&D in the last twenty years--maybe longer. I didn't play it in 4e because (for whatever reason) it didn't feel right, and my friends who I had played 3.X Eberron with were all long distant by the time I realized it would be okay to play 3.X again. In the same way that Birthright makes PCs special, Eberron encouraged a genuine campaign-building dialogue with my players (Who wants a dragonmark? How can we make its expression a story worth telling? What secrets do the other PCs who are not dragonmarked have?). And like Dragonlance, Eberron was/is
classy. Each race, class, and organization is familiar
enough to not incite panic, but different enough from the norm to be exotic, enticing, and ultimately exciting! Hands down the best long-term campaign I ever ran (I think I've run around a dozen long-term?) was in Eberron. When that campaign wound down and came to a close, it was because in RL the group was moving away to all kinds of crazy places; those players can still remember their characters, favorite moments, etc. and ask about running another Eberron campaign. Wildest part is, that was also my
only campaign I ever ran in Eberron.
I would be thrilled with a Forgotten Realms Campaign Setting, but I would probably geek out even more if it was Eberron.
Having said all that, I completely appreciate where the Greyhawk brigade of fans is coming from. I feel like I missed that bandwagon by about a month (no exaggeration). If I had starting playing D&D one month earlier, I'd probably be a Greyhawk groupie. So yeah, more power to you, Greyhawk fans. I hope we all get some love from the powers that be!