A lot of the bloggers around this neck of the woods have long-established campaign settings. I have also met many gamers in real life who have been using the same setting and history for years or decades, be it a favorite published setting (often modified) or something entirely homebrew. I also find that those DMs who have changed the system (added classes, etc) tend to keep these same idiosyncrasies for most, if not all, of their D&D campaigns. (Or Runequest, or whatever their game is)
I find that every time I run a new campaign of Dungeons and Dragons (regardless of what edition, and I have run pretty much all the editions except for the brown books, including WotC D&D) I feel compelled to reinvent the wheel: the setting, the metaphysics of magic, the cosmology, the deities, the role of the various common races, what classes are available, etc. For me, the joy of inventing an entirely new world and all the assumptions that come with it is part of the fun of being a DM. To date, the only setting I've reused was the one I invented for my middle school AD&D 2nd edition campaign. I reused the setting for a "next generation" type campaign I ran in high school, and I used the setting one final time for a 3.5 game played with an old friend who was in town and had played in both campaigns.
As I have mentioned, I will be starting a new campaign in just a few days (actually a bit less than 48 hours.) I have decided to start my game with Keep on the Borderlands. I'm not sure if there are any variable editions of the module, but the setting around the keep is totally generic. (There is a "Realm" which the Keep is on the Border of, and that Realm is beset on all sides with Chaos, but that's about all we get.) I think I'm going to build my setting as I go along for once. Yes, there are elves, but I'm not going to worry where their homeland is or who rules it or what state it is in just yet. I'm not worried about the King or neighboring kingdoms (and in a world without mass media or widespread literacy, it is quite likely that PCs might not even know such information)
What can I say? This is an experiment, and I haven't run anything since the regrettable collapse of my campaign back in May. (Which, by the way, I am still ever so slightly bitter about)I am ready to get going again, and I'm ready to try out a lot of new things. If I enjoy this even half as much as my previous campaign, I will consider it a great success. I do have an eye on the land-owning eventualities of Cyclopedia-style D&D, as I've never actually had players settle down in a campaign before. (To that end, I have made a recent inquiry to Alexis regarding his trading/commodity system, and I feel like I at least have some idea of what direction to go.)