As always take with a pinch of salt...
They're expanding the current story line greatly, and including much of the inner alliances, distrust and full fledged war within the 9 Traitor Legions. References to the Black Legion and their "arrogance" will help better paint the picture.
Models...interesting that a previous poster (i forget their name) mentioned Cult Terminator Shoulder pads...because i've recently been told that there is a NEWISH sprue to be released along with several boxed sets of Chaos Marines. The ability to build any Legion out of the basic NEW boxed set along with Legion specific sprue sets which are to include just heads and shoulder pads. I also hear that the Terminator version will also be available, but not sure if they'll come with power armor sprues. Suppose to be Direct Only.
I got a lil' more news on the upcoming Monstrous Creature...can have Marks of the Gods, but not all Legions can take it. Apparently Night Lords and the Alpha Legion prefer more subtle and stealthy approaches.
Death Guard gain a new ability (maybe Chosen squads only ?) that allows their poisoned attacks to ignore Armor Saves on a roll of a '6', much similar to Rending without the extra Armor Pen.
Daemonic Mounts actual are useful now! The undivided version will be "worth while" for the non-Marked Legions.
Thousand Sons will be VERY pleased with their new found speed.
Iron Warriors become "siege specialists" which equivalents to Tank Hunters and reducing cover saves on enemy units!
Word Bearers are "zealots" and benefit from Chaplains as well as Turn 1 Daemon Summoning. New Daemonancers models are suppose to be SICK looking, but I believe just unit champ upgrades.
Emperor's Children have a haughtiness to them, but prideful, which may give them counter charge (on top of their already higher Initiative). More heavy weapons upgrades per squad, and their Chosen will be...scary fast!
Alpha Legion apparently have the greatest access to Imperial style vehicles, as well as more of an unlimited Force Org slot selections, next to the Black Legion. Much of their Army will be able to upgrade to "Infiltrators".
Black Legion, aka Sons of Horus, aka Luna Wolves are tentatively stubborn, and have greatest selection amongst the units/vehicles. HOWEVER, they're not fully trusted, not sure how this works out in game terms.
World Eaters truly become the combat monsters they're story lined to be. Marked Dreads, and Daemon Engines will be a prime staple in most Armies...
I can't say that the Thousand Sons basic troop will get heavy weapons per say, but their new psychic upgrades will pretty much "counts as".