Back to the Daemons: thanks to a large number of their HQs having Wings and with their Daemon Princes having access to them they are able to get a large number of Flying Monstrous Creatures (unlike their Chaos Marine counterparts) which has been dubbed the Flying Circus as this build relies on making their big guys hard to hit and also enables them to fly around torching units with Breath of Chaos before gliding down to assault units later on. As the lads over on the 11th Company Podcast said, when someone says a Flying Circus army is not that good he's a Nid player but when someone says they're really good they're a Daemon player.
What's not to like?
Well, the HQ's with Wings are pricey (200+ pts each) while the Wings upgrade for Princes almost double his base cost so getting a lot of them prevents you from getting a lot of the other units, some of which are no-brainers now unlike in 5th, where most lists were of the Fatecrusher variety take advantage of wound-allocation in order to keep units alive long enough to get into combat.
So here's a sample Flying Circus list for 1500 point games, seeing as that is where our games are starting until we can get the hang of rules.
3 x Daemon Princes with Breath & Wings & Mark of Tzeentch
3 x 195 pts = 585 pts
So you've got 4 flying beasties with multiple wounds, Eternal Warrior and Breath of Chaos with re-rolls to their saves if they do get grounded (providing they're within 6" of Fatey) that should be very hard to kill off and make it very difficult for an opponent to get the VP for Slaying the Warlord, assuming Fatey is in command. The main problem is that you've had to spend almost 1000 points to do it leaving relatively little for units on the ground (582 pts) that will actually win you games, as armies do still need to have plenty of Troops to see them through missions, assuming they're BRB ones.
With those points, I'd probably do the following:
3 x 5 Plaguebearers
3 x 75 pts = 225 pts
7 Flamers with Pyrocaster
Tzerald on Disc
That is not nearly enough stuff. The Plagues are not as tough as they used to be by any stretch: most cover has dropped to a 5+ apart from Ruins; their FNP has dropped to a 5+ but at least they get it against Melta, Plasma and Combat Weapons (as long as they're not S10) and their small unit size means they're likely to be susceptible to being wiped out by focused anti-infantry fire and combat units, which there is more of in the game now. As a result they'll have to be played very defensively, but that hasn't changed massively from before. It does mean that if caught they're probably dead as opposed to tarpitting for any length of time, even taking account of the changes to Fearless. There's also the fact that they are no longer able to move more than 6" per turn due to Slow and Purposeful so they need to drop nearer to objectives now and will typically take 2 or 3 turns to get there if they have a bad scatter roll. There is also the fact they don't actually kill all that much.
Flamers, post-WD update, I have found to be the new workhorses of the Daemon army as they've got the tools to take care of almost any opposition barring huge hordes thanks to their Breath of Chaos attack and the new Wound allocation rules making them a very dangerous prospect to be anywhere near, let alone assault and it has helped point me toward a list that puts as many models as possible on the board. (More on that later). They can soak up lots of anti-infantry fire but one squad on its own can only take so much punishment before it gets destroyed, as I found out this past weekend where Guard lasguns and Scarab Swarms put enough hits on the unit to thin their numbers significantly enough that they were no longer a huge threat. That really means you need more than one unit for them to be most effective because you have to take Deep Striking Mishaps into account, the short range of Breath (which is what you want to fire every turn) and the fact that their save just isn't that good. Of all things, it was it's save got worse in the update - maybe GW wants stuff to die in 40k a lot more?
So how does the above affect the list? Well, it probably won't work very well as that one unit of Flamers will get swamped before they get a chance to deal out damage - why shoot 6's to hit Flyers when you can hammer the 5+ save burning-monkeys on the ground before they get to nail you with lots of those Flamer hits? Once they're gone you can start to hammer the Troops that can't get away. It's not great to run that at 1500 but maybe it's in with a shout at 1850 (the size of armies we ended playing in 5th) where you can get another unit of Troops and another unit of Flamers with a Tzerald to dive for objectives and take up some of the slack when it comes to killing/soaking up damage, respectively.
The army does have problems when it comes to dealing with Flyers though as Flying MC's don't have Skyfire so their shots (Bolt of Tzeentch) hit Flyers on 6's and Vector Striking them is limited beyond using a Bloodthirster to do it, and that's an expensive toy to be relying on just for that especially when he's so good at taking out Vehicles and Characters. So we'll have to re-jig it.
2 x Flying Princes
2 x 7 Flamers
Ugh. That's just enough for a few Plaguebearers. Fatey might have to take a hike in this build as he's using up too many points at this level. Even the Princes are relatively expensive for what they do, as are the non-Herald HQ choices. There are other options open to Daemons that don't involve Flyers and that led me to another line of thinking: Spam.
Flooding the board with Bodies making it very hard to get away from and put down, as there are so many there. I was trying to nail something down when I came across Hulksmash's ideas on They Shall Know Fear. I'm not sold on the idea of Horrors as the only Troops as Plaguebearers, despite their problems, are tough enough to still have their place in a list and you can get 2 units of 5 Plagues that hide behind a wall/LOS-blocker for less than 10 Horrors, but I do understand the need to have the large units of Horrors (so they don't get murdered too easily and can kill stuff with weight-of-fire).
3 x Tzerald on Disc
3 x 65 pts
3 x 7 Flamers with Pyro
3 x 166
2 x 5 Plagues
2 x 75
9 Horrors with Changeling
2 x 5 Screamers
2 x 125
2 x 3 Seeker Chariots
That's 1491 pts.
Damn. I really wanted Nurglings in there as they're a brilliant tarpit unit thanks to Fearless (no wounds from Combat res now), 5+ Invul and Eternal Warrior, which forces opponents to pour huge amounts of firepower into dealing with them even if they take double wounds from Blast/Template attacks. They can also be used to pin units down and allow the Chariots to smash into them in a later turn, hopefully unharmed as the Nurglings were the targets of enemy firepower. There's also the silly Epidemius/Grot combo to get them maxed-out Tallyman buffs straight away but that requires you to burn up ally slots on Grots and not Plague Marines and other Mark of Nurgle units to take full advantage of the Tallyman table, which seems counter-productive. That's also assuming that Aura of Decay is able to punk out your Allies of Convenience in the first place.
One option might to the above might be to drop the Flamer units to a smaller size and use the remaining points to get another Seeker Chariot unit, or get the Nurgling tarpit I'm after but that might mean that units are too small and too easily blasted off the board, as well as being too limited in the amount of damage they can potentially deal out (you will want 7 Flamers to make sure a 3 Hull Point tank is wrecked on average).
Why Flamers with the Pyrocasters? For more Wound allocation shenanigans, of course. You can put the Herald at the front of the unit and the Pyro to the rear or the side of the unit so you get to close down your opponent's opportunities to bypass the source of LO,S rolls in subsequent movement phases.
Seeker Chariots? I reckon they're solid enough to run in units of 3 and there are enough other targets for enemies to worry about that at least one unit will get to make it into combat and deal out enough damage with its Thresher-thingy to wipe units out wholesale.
No Fiends or Bloodcrushers? No. They must both reach combat before they can start doing damage whereas the Flamers can do it from a distance and put the fear of God into units that want to assault them as they'll often be ruined by overwatch fire, whereas the other two have nothing to stall an impending charge. At least Fiends have Hit and Run to get away from Dreads and tarpits, though. I reckon that the WD update has done a lot to buff Flamers beyond the former traditional Elite-slot favourites. Funny how that happened with the release of new models.
These are all relatively early thoughts, mind, and are susceptible to change. I am expecting to have to revise this after GW put out their next round of FAQs which are expected to be released at the same time as their new starter box set.