TYRANIDS
The Tyranids will be only AI-controlled (Source)
The Tyranids will always try to eat away some of your territory and players in the war council or in a strike force will have to manage that aspect of the game. The Tyranids will stay away from large battles. (Source)
The Tyranids will be there in order to keep the game somehow balanced. The most powerful faction will attract a lot more the Tyranids and will need to protect their borders a bit more (this will allow the other races to gather their strength and provide a real challenge) (Source)
Tyranids won’t be the only NPCs to fight at launch (Source)
Units: Carnifex (Source)
CUSTOMIZATION
You will be able to customize your spaceship (change its appearance, equip it with artillery pieces, etc) (Source 1, Source 2)
Players will be able to modify their look but only on certain parts of their character (Source)
Strike Forces and Squads will be able to adjust their look with various armor pieces and decals. Every faction and sub-faction should have paler and darker versions of their skin colours as well as some facial traits variations. Players will also have the option to remove their helmet (Source 1, Source 2)
It won’t be possible to re-skin your character with sub-chapters colors at launch (Source)
All the players that participated in a campaign will receive a medal to put on their armor (Source)
Players will be able to upgrade their weapons, but probably won't be able to change a weapon into a different one (for example, if you want to shoot farther, you would attach an upgrade to your barrel) (Source)
Eldar players will have the option to choose a male or a female character but Space Marines and Chaos Space Marines will all be male (Source)
Augmentations (think of Cybork Bitz or Mechadendrites for example) will be either accessories or properties of a class (Source)
You will be able to customize the HUD in order to have the level of info needed for your class/role (scaling of HUD elements with some layout options/adjustments, turning individual elements On and Off). The dev team plan is also to strike a good balance between an in-helmet feel and a minimalist look (curvy/tilted elements and augmented reality-style markers, plus some effects like noise/static, etc) (Source 1, Source 2, Source 3, Source 4)
HUD mods may not be supported at launch (Source)
Players will be able to create different gear sets and load them quickly without going to their inventory/character sheet (Source)
MISCELLANEOUS
There will be some kind of trading but it’ll be limited (no trading between Space Marines and Chaos Space Marines for example) (Source)
Eternal Crusade will feature a craft system which will allow you to enhance your gear with new technologies (Source)
An application will be available to record kills in order to post them on websites like Facebook and Youtube (Source)
There will be four different types of environments on the surface of the planet: desert, jungle, arctic and temperate grasslands. However, there’ll be stranger environments in the Underworld (Source)
The world will be bigger than Skyrim or Planetside 2 (Source)
No confirmation yet if ragdoll will be implemented or not in the game (Source)
Some iconic characters of the 40K universe will make an appearance but they won’t be playable (Source)
There will be some important NPCs communicating with the factions (though they won't give any “quest”

(Source)
There will be achievements (achievements for exploration will also be present) and they'll unlock various accessories, weapons or hero classes (Source 1, Source 2, Source 3)
There will be an animation in front of your screen when you get Achievements but you'll be able to turn them off and keep only the corner notifications (Source)
There will be a night/day cycle (Source)
The devs will be taking part in battles (Source)
An integrated voice chat may be implemented in the game, but there will be quick voice commands for sure (Source 1, Source 2)
Text chat channels will probably not work cross faction. There will also be different language channels (Source 1), (Source 2)
Oculus Rift won’t be supported for launch (Source)
The dev team is looking into voice modulation so that all people playing Marines would sound like Marines, Orks like Orks etc (Source)
There will be no queue to get on the servers or to get into a fight thanks to the PikkoServer technology (Source)
Players will be able to use many different emotes (Source)
Dead corpses maybe won’t stay on the battlefield due to performance issues (Source)
NPCs probably won’t have voiceovers for launch (Source)
There won’t be playable Titans at launch (note that Titans may not appear at all for all we know) (Source)
Each faction should have a distinct UI (Source)
The dev team is taking a UI for the color blind into consideration (Source)
There will be a tablet/phone application to help the members of a community/squad/strike force stay connected together and be helpful even when out of the game client. You can expect at least loadouts and inventory access (Source)