That's an interesting question. I've run two home campaigns since I started 3.x and both of them were in the same homebrew world. (Though I moved the second campaign a long ways back in time and adapted/expanded a few adventures I originally wrote for Living Greyhawk to run in it).
I've certainly played a number of published settings though. (In Living campaigns and a variety of computer games). What would I be looking for if I decided to run a game in one? Let's start with a few possibilities that I've thought about and offered to the gaming group:
Greyhawk--status: I've got a fair amount of Greyhawk material
Its attractions-I'm familiar with the world through the RPGA's Living Greyhawk campaign and the supplemental materials I've read when writing for it. It's a solid fantasy world that will support the core rules level of magic and a variety of play styles without a lot of adaptation.
Adventures-I don't know that I would necessarily used published Greyhawk adventures in a home campaign, but one of the attractions of Greyhawk is that most generic adventures could be fairly easily adapted to the world. Goodman Games' Dungeon Crawl Classics feel right at home. Dungeon magazine adventures like the Curse of the 7th arm and independent adventures like the Dead of Winter would adapt pretty easily.
Arcanis--status: I've got the Player's Guide to Arcanis and Codex Arcanis
Its attractions: I'm familiar with Arcanis from the Living Arcanis campaign and it has mechanics worked out that enable the mechanics of the game to support its unique flavor. Also, it seems to enable my style of play quite well.
Adventures: If I were running an Arcanis home game, I'm not certain I'd run out and buy Blood Reign of Nishampur or any of the other Arcanis adventures available. I think I would want to develop my own plot along the same general lines as the plot Paradigm is advancing in the Living Arcanis campaign, but with no guarantees that it would reach the same conclusions when all was finally revealed.
Other supplements: Player's Guide to Arcanis and Codex Arcanis are definitely selling points, but I'm not sure that the forthcoming books like Magic of Arcanis or Jungles of the Mind would be selling points. Since I would be creating my own plot and filling in the blanks--possibly in a different way than Paradigm does--and I certainly wouldn't want to pre-approve any mechanics Paradigm comes up with, the continuing support for Arcanis might actually be a net negative.
Forgotten Realms--status: I don't have any FR material
Its attractions: The biggest attraction of FR for me is nostalgia. I liked playing Curse of the Azure Bonds on my C-64 and Baldur's Gate and Neverwinter Nights. There's a ton of material that I'm familiar with and could draw upon to create stories.
The Adventures: FR is like Greyhawk in being very easy to adapt generic adventures to. That's an advantage. The hoard of FR adventures would be an advantage too if I were convinced that some of them were good.
Supplements: FR is vast enough that the supplements are probably a selling point. I could buy a supplement on the Sword Coast or look up stuff online rather than making it all up. That's a selling point. That players might assume the mechanics in the book were available because it's Forgotten Realms is a disadvantage. (No Peerless Archers or Persistent Spell, thank you very much).
Scarred Lands: status: I don't have any Scarred Lands material
Its attractions: I've not looked into Scarred Lands much but I've followed a few storyhours set there and I liked the flavor of the storyhours so I might well like the setting. It's got a fair amount of material out there for it as well (though since it's discontinued, I'd need to do some looking to find it I suppose). Another attraction: it's officially discontinued so I wouldn't need to worry about evaluating new "official" mechanics or future adventures incorporating elements incompatible with my developing plot.
Distilling those thoughts, I think that the primary thing I would be looking for in a published setting is the setting itself along with mechanics that support the flavor and history of the campaign. If the necromancer king had armies of zombies, I want stats for the feat or item he used to exceed the normal limit on controlling undead HD. If dwarves are cursed giants in the setting then I like having mechanics that reveal the effects of that curse. Prestige classes for a battlemage of the Coryani legions or an Ymandragoran harvester are also nice, as are some variations of the core classes that support the flavor of the campaign world. (Of course, if the mechanics are bad then it's no good--I really should review the Player's Guide to Arcanis and go into the good and bad bits in more detail). Similarly, details about the products of the various provinces and such are nice.
Adventures are more of a mixed bag. I probably wouldn't choose to run a lot of adventures for any given setting, so having a long adventure path is not necessarily a good thing. (And, if I were running an Arcanis campaign, it wouldn't be set in Canceri so the published Nishampur adventures don't really attract me either).
So, summing it all up, I think that one-shot campaign settings seem fairly attractive to me; I don't need regional supplements and such, but regional and rules supplements can be nice too. I would probably be interested in a setting with NO published adventures at all, but a good adventure that goes with the setting would probably be a selling point as well.
