Taken from a post on 40k Forums. Comments in red are mine.
Now that everyone has ordered their redundant Death from the Skies it is time to move on to the next "impeding" release: Deamons. I am one to think that they aren't coming out just yet, but maybe with the April release. With rules rumors being hard to come by let us look at some obvious ones right in front us and others not so much.
As most people realize Daemons will lose both Fearless and Eternal Warrior replaced with 5+ invulnerable and Fear. Eternal Warrior never really made sense in the grand scheme of things anyway because wouldn't Force Weapons be perfect to "banish" Daemons back to the War? As for Fearless,
see a return of "instability" which is going to be close to 5th edition Fearless.
This makes more sense to me as EW was too much. Also now Daemon gives them immunity to fear anyway. Bringing instability back would be so much better. Here's hoping that is true.
As I stated in a previous rumor GW is hell-bent on having each army hate something. Daemons are no different they just seem to hate themselves now.
Taking a look at the Chaos Space Marine and Warriors of Chaos books you can see where this is going. Each Chaos God units will hate a certain other god. This leads into the next rules change right in front of us. Depending on the Daemon's God it will determine the special abilities they get.
Good. Bring back the original hatreds. In reality Khorne would never ally with anyone, except Nurgle if the old plague guys didn't use pyskers, Slaanesh would only ally with Tzeentch and Nurgle. Tzeentch only with Slaanesh, and Nurgle only with Slaanesh or as said above Khorne if the cercumstances were right.
Khorne: Furious Charge
Slaanesh: Rending, Run bonus, Fleet
Tzeentch: Re-roll 1s for saves
Nurgle: Slow & Purposeful, Shrouded
Other changes that will surely happen...
Flamers and Screamers are getting nerfed, but the real big change will be Deamons having standard Deployment rules: no more Daemonic Assault.
Yay! Flamers and Screamers were horribly overpowered and still are even with the rules update. Flamers for a start should not be AP3 and Screamers should be killed if they explode a vehicle.
The big reason Deamons were fast tracked has to do with Alessio Cavatore. Chatting with a few inside GW there has been an everything Alessio purge, and Daemons are the last big chunk of Alessio material left. Yeah, I know it sounds kinda petty, but it was a major impetus to get Daemons done. Daemons will still primarily be an assault based army, but that is the nature of the models having to be used both for Fantasy and 40k. That means Daemons will rely on speed to get across the board. Expect Chariots, warp portals, beasts, cavalry, jump troops to help with all this. Then there is cover saves Nurgle is the first piece of the puzzle, but just like certain Dark Angel things expect a "Clouds of Flies" to provide bonuses to cover saves all around. Chaos Space Marines and Deamons ICs will be able to join each other as long as their gods match up.
Cavatore made the best version of the rules and now he's gone they are dropping everything.
Anyhoo, dropping the deepstriking daemons element is a great idea. It never really worked the way I thought it should. Proper deployment might hurt them a lot though. We'll have to see. Will also have to see how it will change the Daemon missions from Battle Missions.
Things have been spares from all rumor mongers, but a lot has to do with GW "renewed" lock down; translated GW rumor monger wack'a'mole.
Another good thing to understand is GW is moving quicker with releases than ever before. They had almost an entire year to create a backlog on sculpting and rules. To give you an example, the Chaos Space Marine codex was done in December of 2011. I am not talking about just the rules, but everything the photography, the layout, everything! Dark Angels same thing, done in March 2011 [I think he means 2012 -Sour]. That means Daemons have been done since June and Tau and Eldar compelete or almost complete.