I really wanted to vote twice: Eberron and Dark Sun. But I voted Eberron.
It bugs me that Dragonlance is in the lead. There's no point in making the next available setting another "standard fantasy" setting when that's what FR is. Greyhawk, at least, has the old-school D&D and nostalgia factors going for it.
I'd readily buy Eberron and Dark Sun material. I might buy some Greyhawk material. I doubt I'd ever spend money on new Dragonlance stuff.
It'd be cool to see more of Wizards internal polling to check out the relative size of each age category who completes the online surveys. And how many folks are new to D&D, versus players who have made the jump from previous editions.
Then again, to look at it as accurately as possible, that would still only tell us the ages of the people choosing to complete the surveys...
Of those settings, it's the one that currently appeals to me most.
I'm fond of Dark Sun, though, and Greyhawk would be cool.
Dragonlance....I just...eh. I'm playing a DL game right now and it's not a setting I love. I don't hate it, but there's too many one-dimensional character types (Kender 1: "Hi, I'm a Kender, my defining trait is that I'm childlike!"; Kender 2: "I, too, am childlike and filled with wonder!"; Kender 3: "Unlike those other two, I am very childlike!"), too closely tied with the One Big Story to have much diversity, blah blah blah.
We also had Tyranny of Dragons which is close kindred in high-level plotline.
Greyhawk has a Moorcrockian Low Fantasy edge that I can get into as a counterpoint to the shiny happy Forgotten Realms, and Dark Sun is pretty much always welcome, but Eberron is more my interest.
Whatever setting wins means very little. But what it does show is where the interest is at. I mean this is a 24 poll on twitter, not mentioned on the official website. Pretty lame as a thorough research technique. But if they are intending to do Eberron or DL, they know there is support there. Not so much for GH.