The short version:
BACKGROUND
If you remember the 'Storm of Chaos' campaign, forget that. Time was wound back, and that never happened. I'm quite serious about that- it simply never happened.
At the end of 8th edition, Games Workshop released a series of 'End Times' books, in which Chaos (led by Archaon) invaded the world (aka the Storm of Chaos, Mk II).
There's lots of politics, alliances, etc. and plenty of just straight stupid crap. Nagash is back, Tyrion dies, Tyrion comes back to life, Ulthuan sinks, Tilea, Estalia, Bretonnia and the Empire are all wiped out- or at least every major city is. Many, many major characters die- including Orion, Ariel, the Lady of the Lake, the king of Bretonnia, Teclis, and basically every single 'minor' character (and yes, many gods die too). The Dark Elves, High Elves and Wood Elves all ally, becoming one friendly bunch all under the rulership of Malekith, while living in Athel Loren. If you're even vaguely aware of the Elven history that one really hurts the head.
This could have been very cool and climactic, but they completely Games Workshoped it, and turned it into a massive, rage-inducing clusterfrick. Multiple books were released- rule books and fluff books- that were filled with contradictions. Many major characters died 'off screen', being just a part of a footnote in some message bearer's lines, and things like that. Many races had almost no showing- for example, Dwarfs basically never left their holds. Lizardmen flew off into space before things got too hot (not kidding), and Bretonnia was simply everyone's punching bag- they did nothing but die. Khemri was also wiped out by Nagash, single handedly. He then took all of the undead from Nehekhara and the rest of the old world to go fight Chaos.
There was one example of an elf, who was turned into a vampire. She was later redeemed by an Elven god. Tyrion then killed her for being a vampire. Why? The author of the book where she died simply hadn't read the earlier book where she was redeemed. There was no communication among authors, and it showed horribly.
The 8 winds of magic were bound into 8 mortals (or semi-mortals), to try and fight the Chaos Gods. This changes a bit as characters die, but from memory the 8 characters are: Nagash, Tyrion (after he's brought back to life), Teclis, the Everqueen, Malekith (elves were very over-represented), Karl Franz, Grimgor Ironhide and Balthazar Gelt- an ex-Grand Theogenist of the Empire.
The very end to this nonsense is that another warp gate was found in the Empire, opened by Archaon, and the entire world was destroyed. Again, not kidding. Everything. Dies.
The most brilliant part of all of this: It's stated at the end, that with their victory, the Chaos Gods lose interest and turn their backs on what was the world. That's a real sense of achievement for both sides there, when the victor doesn't even care.
NOW
Though not all details are known, Age of Sigmar is set in a new world. Don't know where it came from, but it's basically a chaos-covered daemon world. Somehow the 8 winds of magic survived, and those who were tied to them come back to life as gods of these little bubble-realms. Karl Franz is Sigmar now. Now those forces are sallying out, trying to kill Chaos and claim more territory. No idea why really- there doesn't seem to be an end goal yet (they can probably never beat Chaos).
RULES
On the positive side, the Age of Sigmar rules, and updates to use all current models in the game, are free. They'll be up on the GW site this Saturday.
The negative side- everything else.
The rules are a 4 page pamphlet. The holes are gapingly obvious and horrible. Even two people who want to play a proper game are going to encounter bits they struggle with. Issue #1 is that there is no balancing system, at all. No points, no force limits- it literally tells you to place as many models as you want, of whatever type you want (mix and match armies freely), until you want to stop or your deployment zone is full.
Tactics and strategy seem to be lost altogether- there's no flanking, no charge bonuses- nothing at all to encourage careful manouvering. Models have 4 stats- Movement, Bravery, Armour, and Wounds (2 or more wounds is now common). Weapons have stats as well- Attacks, To-hit, to-wound (both of these are set values, regardless of opponent), Rend (a penalty to the enemy's save) and Damage (multiple-wound weapons are also now common).
Models are now all on round bases- not that it matters. Bases are irrelevant, as all measurements are made to and from the models themselves. Any part of the model- so you can kill a pikeman by attacking the tip of his pike.
Models are also much larger- one 'new race', commonly called 'Sigmarines', are basically angels of Sigmar. They are space marines, in almost all ways- including looks. They stand about as high as an Ogre.
One of the big aims is to make things copywrightable. Forget anything that looks like Bretonnia or the Empire- they're too historical. All new models will look 'unique', and over-the-top. Names have also changed- No more Orcs, Goblins, Elves, Dwarves- now they are 'Orroks', 'Grots', Aelfs' and 'Steamhead Duardins' (yep, poor Dwarves).
In short, did you like Warhammer Fantasy Battles?
Too bad. It's dead, and its ashes have been desecrated by this monstrosity that's barely even playable.
There's nothing concrete at the moment, just rumors.
Sorry Dave, but rumours no more. I can provide links to the 'full rules' (hah!) if it's allowed, and anyone wants.