I think the Oathbound campaign setting by Bastion Press (which was mentioned above) definitely has staying power. The setting is one of the main focuses of the publisher, receiving regular support. I think the biggest thing going for Oathbound is the commitment by the publisher to support the setting. Examples:
1. The core setting book is now available as a PDF, giving a wider audience access to the book.
2. Future books in the line, starting with Wildwood, will be written in such a way that they can also be used as a stand-alone campaign setting in their own right. This way, Oathbound junkies have their next Oathbound book, but someone who's only interested in Wildwood can play in Wildwood without the need to have ALL the other books. I think this can be a downfall for other campaign settings.
3. Bastion has put out a number of free adventures online (which are quite good), including an introductory adventure meant to pull characters into the world of the Forge. They also have a column, the Raven's Report, which details people in the Forge, often giving some tidbit of gaming "crunch" along with the fluff. The recently released Legacies of Oathbound #1 offers and adventure and an expansive treatise on one of the major bloodholds in the city of Penance. There is more coming down the pipe as far as web support as well.
In my opinion, #3 is one of the major aspects that make Oathbound so appealing. Bastion goes out of their way to support the setting with regular updates, both free and for a modest cost (Legacies is a 140 page PDF for $5.99; that strikes me as fair). As a DM, the adventure offerings are one of the major reasons I selected Oathbound as my campaign setting; I often don't have time to write up my adventures. Tweaking existing adventures is easier for me.
To sum up: I think Publisher support is a major factor in campaign setting longevity.