In fact, being that I'll be playing Orkz, I'm after Ork vs Ork battles between different tribes, cos you kno' us Snakebites is reel Orks, d'os Bloodaxes is almost humies.

I fully agree, Dark Millennium Online was total BUNK.

But Eternal Crusade shows promise, there's no signs that they'll go the route of Order vs Disorder here. It's going to be a world where there is Only War. So Space Marines will fight the Eldar, Chaos and Orks & vice verse.

There have been mentions of temporary alliances. So the Eldar may "ally" with the Space Marines for awhile, before returning to the norm of killing the xeno.

Having Order vs Disorder is rather stupid. All of these races don't get along, and the very VERY rare chance they work together it's for a scant battle then it's back to being enemies again. You wouldn't see ANYONE allying with the Necron, and the Tau would only ally with people to be all "Hey, so can I talk to you today about our lord and saviour, the Greater Good?"

Having battlefields where everyone fights everyone is going to be so freaking epic it's not funny. Is your force doing horrible while the Eldar/Tau are doing decent.....EMPEROR PRESERVE ME there goes a bunch of Orks and Tyranids accidentally saving the Space Marines or what have you (can go for any faction in any order,) due to all the cross fighting.

This also allows for interesting tactical alliances between REAL people on opposite forces. Makes it less artificial and makes you have to be wary of backstabs or ambushes.

The block was called Destruction forces, not Disorder.
I hated and still hate such approach in either world of Warhammer or 41st Millennium Universe.
Such a rich and detailed lore deserves more than simple coping of casual WoW mechanics. If developing company isn't able to understand and/or realize that fact - it does not deserve to work on the wh40k based MMO in my opinion.

As I stated before, 2 sided pvp sucks.

But there are people hoping for alliances and I write this to give you hope. There are people who know each other playing separate factions. In certain battles I would imagine arrangements will be made for "friends helping friends" to cause a little chaos. I wouldn't be surprised if while you were assaulting an Eldar fortification as Chaos that your rear line gets harassed by a small contingent of Space Marines or someone else who happens to be buddies with the guys you're assaulting. All of our friends are only a TXT or PM away in the digital age...

Two sided PvP worked in WH Fantasy, sadly it doesn't really work too well in 40k simply because GRIMDARK

Order and Disorder is pretty stupid for the 40k setting. Like others have been saying, very temporary "alliances" may occur (this is incredibly rare in the 40k universe) between some factions only, like Space Marines & Eldar or Orks & Chaos, but that's it. And they're not even true alliances. It's more like two factions just agreeing to not to fully attack each other and both focus their attention on another faction for a short period of time. Then they go back to killing each other.
You would not see the two forces actually fighting together on the battlefield. No Space Marines are going to form squads with Eldar and no Chaos Marines are going to form squads with Orks.

No. It is not bound to happen and it should never be. Space Marines would NEVER work with Chaos Marines and vice versa. Not under any circumstances. It goes against lore 100%. They are eternal mortal enemies and it should always be that way in the game. Period.

I almost liked the way WA handled Order vs Disorder. IG sort of allied with Eldar but not really, then gave 'em the finger and let 'em all die like two missions before the last one.