Time for some initial thoughts:
1. Glad to see Screamers and Flamers toned down. I have included them in my army and played against them in their previous version...they needed a fix. For those that have bought 27 of each and are surprised this has happened...what can I say other than you knew this was coming and spent your money anyway.
2. The codex has shifted from an elite army to a horde "ish" army. I like the change and variety.
3. I like the fact that you can benefit from "Mono-God" armies and that you do have ying to your yang with the opposing forces of chaos.
4. Your going to have to buy more models....yeah go figure. Again this is not new. So...GW can make units way OP so the model count is few (Did we like GK's in 5th?) or we can make more variety and lower the point cost so you can include more units and have built in redundancy. In other words you can make some of the people happy some of the time, etc. This akin to the negative comments/discussion like this: "Yeah, GW makes a new unit and makes it good so I have to buy it...they are so money hungry! Evil GW!" Ok.....well did you want some new units in your new codex...."well, yeah"....and would you like these new units to suck? "well, no, I want them to be good."....Ok, then what actually are you complaining about?
5. Nurgle for its resiliency and Slaanesh for its kill all/speed stick out early to me.
6. The Nurgle and Slaanesh psychic powers are almost all good and the novas are very good. I see a IG psychic choir and the Slaanesh Initiative Test Nova being a very hard counter to every blob, infantry heavy, or death star in the game.
7. I can see IG and Deamons making a great allied combo, which is primary a mater of choice.
8. Deamons and Horde Orks or even better, methinks, Battlewagon Orks making effective competitive armies.
9. 90% or better of the Deamon units can Deep Strike, icons are easy to get and cheap...this opens a flood of possibilities with allied armies that shoot well and have a strong table presence (Orks, IG).
10. All the Nurgle powers are good and they also have a 12" Nova Toughness test bomb. This slapped on a non scattering large scale based great unclean one with a boat load of wounds and high toughness is a potential game changer...though for Nova's the winged DP's are likely superior.
11. The soul grinder is a very solid unit, price is write, comes with flakk, can be outfitted to fill many roles, retains armour 13, 12,12,11, and can deep strike off an icon or solo. Really outshines the Defiler.
12. Deamons will make great and fluff appropriate allies for CSM. Most of the killy in CSM is expensive the deamons give bodies and additional killy, but best yet much mor MOBILITY without resorting to IG or Orks.
Good stuff and still reading,
BDS