I think that any setting at all can prove to be home to a great campaign. Each of those listed in the poll has potential, and if your DM and players are good, you can get a lot out of them.
Each setting has inherent pros and cons, and plenty of hooks to create adventures and characters and entire campaigns.
Having said that, I think that it's a question of the pros outweighing the cons. For me, certain settings are that way, for someone else, it'll be other settings.
My one observation about all the settings listed is that many of them are very Tolkienesque, classic fantasy/kitchen sink type settings. I've played games in most of them, and ultimately they aren't all that different from one to the next. And I by no means think that there is anything wrong with such a setting...I just don't know if most games played in those settings ultimately feel all that different. I don't know if we need 6 to 8 settings that all kind o cover the same ground.
So having said all that, I'm not too surprised to see the results, although I almost would have guessed Forgotten Realms instead of Greyhawk. I suppose that of those two settings, perhaps nostalgia and absence played big parts in the voting.
Dark Sun is the most radically different in many ways, and Ebberon was the main setting addition from the 3E era, so they both make sense. Planescape is the most thematic, along with Ravenloft, so I can see them making it in.
I guess I'm a bit surprised by Al-Qadim. Not because there's anything wrong with it, but more because I thought it actually took place on Toril and therefore counted as Forgotten Realms by default.