So we had a solid character burning session which got the players really literate in the setting as well as a fun starting session that sort of introduced the initial politics and location (basically, saboteurs/war provocateurs attempting to destroy the negotiations of the Thronehold Accords).
The Character burning was a lot of fun and really successful, each character has an inter-related background and each has a Goal and a Trait, which they get bonuses to their rolls if they're pursuing.
The Characters and their goals:
- Kalashtar Revenant Assassin - Attempting to find his murderer and reconnect with his Quori spirit.
- Human Dragonmarked Artificer - To re-unite House Cannith and discover the secret of the Mourning.
- Human Dragonmarked Warden - To find the meaning of the Prophecy hidden in her Dragonshard and in her dreams.
- Changeling Invoker (of the Traveler) - To track down a mysterious saboteur(?) that only he seems able to see.
- Warforged Warlord -To fix the planar curse he's under, and recover the occupied city of Thaliost.
Now comes the tricky part - Building Campaign plots that are connected to the characters and working out NPC groups and their motivations.
I've picked not one, but three Campaign plots that are going on at once. That may sound ambitious or overly complicated, but they're actually pretty loosely defined, and the whole idea is that no matter where the players go off from my expectations (assuming the DM credo that no plan survives contact with the PCs) there'll be "something going on". It's really more about having comprehensive narrative back-up if things go awry - I'm under no obligations to use anything that doesn't get related to, after all.
What I've then done is drawn an initial Relationship Map. I've placed all three plots (or their macguffins) in a triangle around the center, and spaced the characters around the edges, showing connections to all the NPC groups and places that are involved. I've tried to eliminate as many dead-ends as possible, so that when the players meet or fight a particular group, visit a particular location, or acquire a particular artifact, all I have to do is look at the map to see where it could lead to or see who's going to want to be involved with it.
Then I hope to put together about a dozen notecards filled with adventure ideas. They could be combat encounters, just a list of antagonists and their motivation, or a quick location with a sketchy map and a trap and a bad guy, or a skill challenge to find out about something. For a while I was planning on getting all this stuff down on my personal campaign tiddlywiki, but for brainstorming I've found it easier to go analog and then just save the online tiddlywiki for "things that have actually happened in the campaign", in order to keep it as a primary database for need-to-know info during play.
Anyway, if anyone is interested in my brainstorming, here are my three Campaign Plots:
The Search for the Ashen Crown - Only vaguely based on the Module, I'm mostly stealing the flavor and none of the adventure. The Ashen Crown is an artifact from the Elven-Dhakaani war that contains prophetic memories from throughout the Galifaran reign, desired by many in hopes that it will grant the power to reunite Khorvaire under its rule.
- Constructed by the undying court, it consists of four parts located in different places or possessed by different people: a circlet, a necromantic dagger, and two polished Dragonshards.
- Currently being pursued by the Royal Eyes (nominally backing the PCs), the King's Lanterns, the Kech Volaar (goblin lorekeepers) and the Emerald Claw.
- Theoretically the starting "linear" adventure of the campaign, as the PCs are sent to the catacombs below Thronehold Citadel (even as Treaty negotiations rage on overhead) to recover the circlet.
The Secret of Whitehearth - Nominally cribbed from the 3.5e Eberron modules, this plot involves the destruction of the Mournland, lost Cannith Schema and a secret Vadalis project to create genetically magebred superhumans.
- The Aurum, through Cannith South, are desperate to uncover the cause of the Day of Mourning for their own ends, competing with the Emerald Claw. Cannith West hopes to unite with Cannith East in stopping this.
- The Lord of Blades is interested in acquiring a Creation Schema from the Age of Giants that would allow him to create mighty Warforged Juggernauts.
- The Ashbound believe that the Day of Mourning is the ultimate price for playing with Arcane magic, and seek to destroy all evidence of such.
The Stormfront Cult - A dangerous new sect of the Dragon Below, these fanatics are being manipulated by the Inspired and worship of ancient Giant magic to awaken a Primordial-like being that will reclaim Eberron and end the current age.
- Spoken of in the Draconic Prophecy, various agents of the Chamber are attempting to stop this from occurring, but believe that it has to almost occur, as it will solidify and renew the purpose of the Gatekeepers.
- Part of the plot involves raising an army of giants to besiege Stormreach and drive Khorvairans off the continent of Xen'drick, in order to consolidate the ancient magic there.
- The Sisters of Sora Kell hold a crucial key in the ritual, but it's unclear which side they will choose.
Anyway, I hope that has provided some entertaining reading. 
i'm not sure whether you have done this, but what really pays off is having the characters build NPCs for you as well. Some from their past, some from the present. That way, you can get a good grip on what they want their surroundings to be. also, let them define the power groups they are related to - you'd be surprised what good ideas they can come up with.
if you have not done so yet, introduce some stuff from dmg2 p.15-25. (collaborative world building) It's a nice treat/toolbox for running a sandbox world, and your players will feel even more participating.
Tagging off of that, what have your PCs given you for backgrounds? What's-his-face above me is very much right in that if PCs tell you about their nemesis (or nemeses) than it's a lot more fun for them, and a lot easier for you.
When I was running my first Eberron campaign using the Forgotten Forge track, I had a Cannith artificer who told me that he had a cousin who was the taking-credit-for-other-people's-work sort. A political type.
Lo and behold, a month or two later, they go into a Cannith forgehold, and one of the workers there was like, "Oh! You guys work for Reginald!"
Instantaneously the artificer went into character, clenching his jaw and acting mad. I took that and ran with it, and made Reginald memorably irritating. I even made a motivational poster for him:
i229.photobucket.com/albums/ee11/Ogiwan8...
Anyways. The point is that even if they give you ancillary characters, you can have loads of fun with them.
anywho, I have to add a note here... would it not be interesting toalso add the Tairnadal of Valenar to the list of folks after the Ashen Crown? it seems like a good way to direct their mysterious goals of warfare/conquest. especially given that the Crown is of Elven origin, and the Tairnadal were the first Elves to take to Khorvaire from Aerenal. in fact, it was the Tairnadal who settled during the Age of Monsters, and warred with the Dhakkani.
the Goblins come in various groups, from the Heirs of Dhakkan (who are further disparred), the Darguun loyalists, and individual clans who just want feudal glory for themselves. they are broken up into so many different factions who don't have as great a unity in their culture or goals as the Elves.
but aye, a limit of 3 is fair enough...