Army wide special rule: I think it’s called Platoon Drill
or something like that. Basically, units from the same platoon may ignore intervening squads when shooting. So enemy units targeted in this way do not receive the 4+ save. Apparently this only works if the Platoon command squad is alive and within 12” of the rest of the platoon. I’ve been told that this is just one of the abilities guard Platoons may get, but only as long as the chain of command is intact (whatever that means). So loosing your HQ Command Squad may be a really bad thing.
Doctrines are for sure gone (I think we all knew this), but have been replaced by more mutable platoons. Instead of Armoured fist squads you can buy Chimeras for a whole platoon. Or buy the whole platoon grav-chutes to give them deep strike, or drop the heavy weapons from squads and give the platoon Scout. Veteran squads become upgrades to platoons (i.e. they are not elites anymore).
Apparently infantry platoons let you attach certain types of support vehicles to them(kinda like back in second edition). Things like sentinels, hellhounds and demolishers can be attached to platoons. They still take up a FOC spot, but become part of the platoon and so will benefit from the Platoon Drill rule.
The Leman Russ can be bought as a squadron so you can get 3 for a single FOC. They can not be attached to platoons however.
Sniper Squads can now infiltrate, get stealth, range finders and all models in the squad have sniper rifles (and BS 4!).
Like the Land Raider, a new plastic sprue for the Leman Russ has been made that will allow plastic demolishers and a 3rd variant (my money is on the Exterminator).
The Baneblade in NOT in the new codex. It just doesn’t work (i.e. overpowered)in games that are not Apocalypse in size.
However Stormtrooper squads can now take Valkyries as transports. Both will be getting a plastic kit. Valkyries follow the rules for skimmers I’ve been told. Again, they want to keep flyers and super heavies out of “normal games” but give people the ability to use them with all the special rules in Apocalypse and Planet-strike (if the ever get around to that.)
This last one makes me wonder what’s they are doing, but apparently there is a new HQ in the book; Inquisitor and Retinue
much like the elites from the current Daemon Hunters/Witch Hunters books.