I've recently been working on a campaign for DnD that I will start DMing hopefully some time this summer. I decided not to go with any of the pre-made worlds. Nothing against any of them in particular, but I prefer to have control over the World (and critically, I prefer that the players not know everything about it). I haven't yet given the World a name, indeed I don't know if I will, but It's been fleshed out quite nicely so far. The PCs start in the country of Cavin, which for twenty years has been at war on-and-off with it's neighbours. No side now has the capability to field the vast armies that they once did, and fighting has been reduced to skirmishs involving smaller groups.
The first three adventures are based in and around a military emcampment at Helga's Hollow, which guards a vital bridging point over a large river. These are very railroaded, mostly because several of the players won't have played DnD before and I'd like to give them a chance to get to grips with the mechanics before anything really complicated happens. However, after that, I hope to allow them greater choice, and so have in many places included alternative adventures that they could pursue.
I have also introduced a mechanic for nationality. Basically, the archipelago around which the entire campaign is set was colonised hundreds of years in the past, and this has prevented the nations being based around a particular race. They have their own languages, but it isn't sort of 'this nation is elves and this nation is dwarves'. For this reason everyone must also choose a nationality, which will effect how NPCs view them and also provide some bonuses and penalties.
I have also not included a pantheon of Gods. Usually in most settings people can worship a God, who is assumed to be in conflict or alliance with different Gods, much like the pantheons of ancient Greece and Rome. I took a slightly different route. There are five major religions, not all of which admit the existance of other religion's deities. Indeed, at least one of the major religions is basically atheistic, and another worships ancestors who are held to be Gods. Due to this, divine magic comes from faith, rather than definite theistic intervention. There is no mechanic to represent that, it's just one of the ways of working around the ubiquitous idea of pantheons (which I'm not fond of).
I don't actually know how the story will turn out, but I have it planned up until the point where the PCs will be level 9 or 10.