The Premise: Eberron is a noir world recovering from a war that tore nations apart and a pulp world where adventurers ride airships to distant jungles to retrieve ancient artifacts. Magical spells power the equivalent of trains and telegraphs and light the streets, but with vast areas of the continent still unexplored, a strong sword is still a must for the bold explorer.
The PCs belong to an organization set up by a veteran spymaster from the Last War. The organization is based in Breland but sells its services to various governments and private interest groups. The intended genre is a cross between the James Bond novels and the Maltese Falcon — typical missions might involve exploring ancient ruins, sneaking into an enemy stronghold to defuse a magical doomsday device, or finding out why an agent stopped sending in reports (was he brainwashed? kidnapped? or eliminated by his superiors for finding out too much?). The intended feel is classic D&D — every session will have combat encounters, the PCs will start out relatively weak and inept and grow powerful, and treasure and character advancement will be important parts of the game.
Here's something I wrote up with a lot more about the setting.
The Time and Players: Not scheduled or signed up for yet. I am shooting for about ten sessions and because this is longer than usual, I'm going to try a somewhat unusual game structure. The campaign will be composed of a series of one- to three-session-long missions. When I announce the mission, I'll give the time of every session in it. You are strongly encouraged to come to all sessions or none for a particular mission, but there is no need whatsoever to play in every mission (the characters are assumed to be working on other assignments and so on). There will be some long-running plot threads, but I'll give the necessary background at the start of each mission.
The Characters: Although the campaign will have elements of intrigue, exploration, diplomacy, and so on, the most common element is still going to be combat — ideally every session will have two to three fights — and with a corresponding focus on going up levels and increasing power. Given the premise, characters should be willing to go on this kind of mission and stick with it. Some of the missions may get into noir-ish moral shades of grey, and if some people want to make evil characters this is a-ok by me.
The Rules: Obviously, we're talking d20 with some Eberron-specific things I'll get into later. The Wikipedia page has some information on new setting features, which I can give you more details on. This being d20, I have enough rule notes and house rules to put them onto a separate page (this also covers character creation info).