Because the Daemon army book is so recent, Games Workshop have not yet released an FAQ to deal with ambiguities in the rules. Taking the lead from Convic, we will be using a pre-defined set of rulings. These are included below for your edification:

 (Based on and nearly all the same as the ConVic 2008 Daemon FAQ) 

GENERAL 

Does Insane Courage apply to an instability test?

No

BESTIARY

Are spells cast by a Tzeentch daemon flaming attacks?

No, unless specified in the spell description (or are Flickering Fire of Tzeentch and/or Tzeetch's Firestorm)

Are a herald's Locus and other abilities (eg Slime Trail) still in effect if the Herald refuses a challenge and is therefore moved to the rear ranks?

Yes, as if a Wood Elf Highborn/Noble in a unit of Eternal Guard goes to the back of the unit, they are still stubborn.

GIFTS

Can the effects of two "close combat" gifts be combined, such as "Axe of Khorne" and "Flaming Blade"?

No

Does Pestilent Mucus force Toughness tests even when the wound suffered is saved/regenerated?

No

What constitutes a lore for the purpose of The Banner of Sundering?

Any spell list, including Gut Magic and Tomb Kings Incantations (and including My Will Be Done).

What effect does the Banner of Sundering have on spells such as Invocation of Nehek and Drain Magic, which appear outside any lore?

Invocation of Nehek counts as part of the Lore of Vampires. Drain Magic for High Elves counts as part of High Magic. Drain Magic for Lizardmen counts as its own Lore.

Can gifts stated to be hand weapons be combined with another hand weapon to gain +1 attack?

No

Can the Armour of Khorne and Obsidian armour be combined with each other and/or the Light/Heavy armour of the character?

No

How does the gift "Daemonic Robes" interact with hits that wound automatically?

Daemonic Robes have no affect in this case - the Daemon is wounded automatically.

Can a daemon with Nurgling Infestation add the Nurgling base to an enemy unit of Nurglings?

Yes, although why you would want to is beyond me.

What happens to an enemy model affected by the Temptator gift if he is in a challenge?

No blows will be struck in the challenge, with the challenge remaining in place.

What criteria other than the 20" range make a unit an eligible target for Siren Song?

The target unit must be unengaged, in line of sight, and capable of completing a legal charge against the daemon.

How is Siren Song resolved against units which are Immune to Psychology?

As they cannot fail a panic test, Immune to Psychology units cannot flee, and must therefore charge.

How is Siren Song resolved against units which are Unbreakable?

As with Immune to Psychology units, they will be forced to charge.

Do attacks from a Daemon Prince's "Flames of Tzeentch" Gift count as flaming?

Yes

What is the sequence for charges and reactions when the Siren Standard is in play?

The unit being charged by the Siren Standard unit must hold. If any other charges against the unit being charged are declared, charge reactions can be declared as normal against these unit(s).

If the unit with Siren Standard is found to be out of range, then the other declared charge reactions are resolved.

IF the unit with the Siren Standard is found to be in range, then the charge reactions declared against other charging unit(s) are changed to hold.

Can a daemon with "Master of Sorcery" choose to use the Lore of Tzeentch, and if so, does it know all the spells from that lore?

Can use Lore of Tzeentch, and knows all spells.

Does the Standard of Chaos Glory affect Daemons on both sides?

Yes

Can a model affect by "Temptator" issue a challenge? If so, who decides if he will do so, and which side will respond?

Temptator is tested first. Therefore, he can issue/accept challanges as normal

Is Noxious Vapours a permanent effect? Do models regain ASF/lose ASL when they are no longer in base contact with the Daemon?

No. Yes, they regain ASF/lose ASL when no longer in base contact.

Can a unit in base contact with a Daemon with Enrapturing Gaze take Leadership tests which are taken by the unit as a whole (such as break tests) on their unmodified Leadership if not all models in the unit are in base contact with the Daemon?

If not models in the unit are in base contact with the Daemon, then the unit can use its unmodified leadership

What is the correct procedure for placing the breath weapon template on an enemy unit when a Herald of Nurgle uses the Stream of Bile gift (or casts the Stream of Corruption spell)? (Assuming the Herald and target unit are on the same level).

Stream of Bile/Stream of Corruption are placed with the narrow end touching the Herald, and with no friendly models being hit.

Is Iridescent Corona applied before or after impact hits?

After Impact Hits

If a unit is within 12" of the Icon of Despair, and is within 12" of its Ld 9 general, but the general is not within 12" of the Icon, what is the effective Ld of the unit?

As per the VC FAQ with Accursed Armour and Helm of Commandment, the units leadership in this case will be 9.

DAEMONIC LORES

The spells "Shrivelling Pox" and "Pavane of Slaanesh" are cast at single models. Do these spells ignore the normal targeting restrictions?

These spells may target any visible model in range, including a character in a unit. They may not target units in combat.

If a Nurgle spellcaster casts Miasma of Pestilence, how does this affect other friendly models in base contact? Can they choose to only attack the models with reduced stats?

They can, but unless the entire fighting rank is affected, friendly models can only kill up to the number of models affected by Miasma of Pestilence, with any other attacks wasted.

Are the Tzeentch spells Flickering Fire of Tzeentch and Tzeentch's Firestorm flaming when cast by a model without "Flaming Attacks", for example a Daemon Prince of Tzeentch or a model with Aldred's Casket of Sorcery?

Yes - although they do not include the flaming keyword in the description, the title of the spell clearly states them to be flaming.

Does a spellcaster that casts Miasma of Pestilence have to be in base contact with an enemy unit for the spell to be cast?

No