Reading through the background of 6th edition and one thing is clear, there are no major changes in the story. The Golden Throne is still failing (lol@corpsegod), the Imperium is still going along the same etc. There are lots of new details added too, I'll summarise a few below, mostly Eldar related as I'm not familiar enough with the minutiae of IoM background to be sure which are new.
-750,M41 The Great Exodus. Six craftworlds turn up to some big swirly space thing in Imperium controlled territory, when the Imperial fleet arrives with little gtfo placards the Eldar are gone, as is the swirly thing and all the 'prime' suns within sixty lightyears have been extinguished. IoM launches a mass evacuation and manages to wave 12% of the population (and 32% of the heavy industry)
-783.M41 Eldar destroy an AdMech explorator fleet, when Blood Angels turn up to ask 'wtf' they find the Eldar and Necrons fighting it out on the surface.
-822M41 Abaddon attacks a Maiden World and gets repulsed.
-920.M41 Eldar pirates attack the escort of a a fleet of Black Ships, capturing one troop transport and destroying some frigates. The rest of the fleet is picked off by subsequent raids.
-999.M41 13th Crusade happens as expected, predictions of it being retconned like Storm of Chaos in WFB proven to be bollocks.
-p196 The single time the IoM has managed to attack a Craftworld directly resulted in the loss of an entire sector fleet so they just don't bother anymore. /eldar fistbump
This last one raises an interesting question, is it referring to the attack on Idharae or the attack on Alaitoc? If Alaitoc, then what happened to Idharae? Had Idharae been retconned? If referring to Idharae (made much more palatable by the loss of a sector fleet in the process) what about the attack on Alaitoc in the Path series?
Feel free to post anymore titbits of new background you find as you read through the BRB. That is, when most of you get your copies over the next week or so.