A member of Warseer who claims to have play tested the new demon codex has posted a lot about the list on this thread. Unfortunately, he's not a native English speaker, so some of what he's saying is hard to read. Still, he's offered a lot of information, which I have combined with the previous rumors and summarized.
Here is a list of the units in the codex and their rumored characteristics:
Blood Thirster - Can Fly and also rumored to have a shooting attack, which seems odd. May be as strong as S8! Maybe because of furious charge?
Great Unclean one - T6, but I2 and slow and purposeful.
Keeper of Secrets - Fleet, Hit and run, and some sort of Lash of submission- like ability.
Lord of Change - Rumored to be the most powerful. Carries some kind of force weapon. Can target three units with three shooting attacks.
Herald - Can have up to two for one HQ slot. Each Chaos God has it's own Herald with tons of options. These were featured in January WD.
Flamers - I've heard nothing on these.
Flesh hounds - Have option to have special character.
Fiends of Slaanesh - I've heard nothing on these, either.
Beast of nurgle - Similar to Chaos Spawn with poisoned attacks and feel no pain.
Daemonettes - Have Fleet and Rending. The plastic kits have caused much debate.
Bloodletters - Have Power Weapons and Furious Charge. Loincloth no longer confers 3+ armor save. The other plastic kit seen around the net.
Plaguebeares - Feel No Pain, Poisoned Weapons, Slow and Purposeful.
Nurglings - Got me!
Pink Horrors - S3 AP3 Shooting attack with option for Bolt of Change. Also has option for special character called "The Changeling"
Furies - i don't know, but I'd bet they fly.
Bloodcrushers - Bloodletters on Juggernauts of Khorne. May only move 6" - I'm not sure why.
Seekers of Slaanesh - f.k.a. Mounted Demonettes
Screamers - Treated as Jet bikes. May also have some kind of killer optional upgrade.
Heavy Support - There seems to be a lot of confusion and contradiction in this area. More on that later...
Soulgrinder - Modified Defiler. Possibly source of "unbound Defiler" rumor.
Daemon Prince - A very customizable choice. Treated with disdain by true demons due to formally human nature.
3 Greater Daemons:One for Nurgle, one For Tzeentch and one Undivided)
4 Special Heralds:One for each God. Slaanesh Herald called, "The Masque".
General rules: All Daemons have invulnerable save, can be placed in deep strike and have immunities versus Instant death. All codex entries - even demon engines- are daemons.
I have to go, but I wanted to post this before anyone else (I hope I can beat the Bell of Lost souls). I haven't thoroughly spell-checked this, so forgive me if I've misspelled any names of any Chaos deities.
Also, if you want to see the thread that started this, it's here.