Tactica – Strengths & Weakneses of Daemons

Many have already written exhaustive Daemon tacticas, and with good reason - they are the best army.  The flesh-scribes and internet-servitors serve well as they churn out paragraphs of why bloodcrushers aren't so bad.  But they are bad, and I've never used a blood crusher.  So I don't feel confident writing about them. 
In the 41st millennium, there are commanders who bandy opinion far from the fields of war and there are those who soak those battlefields in their enemy's preferred circulatory fluid.  I am of the latter sort, so I'll tell you what I've learned.

The primary strengths of daemons -

5++
More than anything, the extreme resiliency of an army that ignores, by default, 1/3 of whatever your opponent does to it cannot be overstated.  We are the horde, and the horde refuses to leave.  A savvy commander will be cursing the earth, of course, and then we ignore 1/2 of it.  If Tzeentch is being targeted, he ignores 7/12 of it.  Other armies have mere +'s.  This means that all the points your opponent has invested in AP are wasted.

++ Cursed Earth - +1 ++ in 12", never don't cast
++ Grimoire - +2 ++ for one unit, sometimes.  Screamers are great recipients because they can Jink.
++ Daemons of Tzeentch (re-roll 1's)
+++ All three - re-rollable 2++'s!

Wasted Points
Your opponent has wasted some points in his list.  Wasn't that stupid of them?  Plasma guns, lascannons, demolishers, power weapons - we don't care about them.  Because we don't have armor to penetrate.  Facing a balanced list, your opponent is already down 100+ points in gear that we don't care about.  100+ points down, you say?

++ Railguns
++ Meltas
+ Power Weapons

Free Points
Summoning, Sacrifice, Incursion, Possession.  The summoning factory takes time to learn to use well, but it pays off once your brain can chug the probabilities.  Easily summoning 300+ points of units a turn means that often, you can end the game with more points than you started.  Ain't that mean?  Oh, and you can summon 12" out, which means you have great -

Board Control
Drown them in bodies.  Your hordes of favorite daemons can usually completely blanket your home deployment zone, which means your opponent will have a hell of time taking your objectives, especially because you're summoning more daemons.  They have a choice of hurting what's behind or what's in front.  What's in front will hurt them, so they make that choice, and oops, those units were free.  All your psykers can run even though they mind-bulleted in the psychic phase.

++ Summoning - Free WC with Horrors, Free Rending with Daemonettes, Free Shrouded with Plague Bearers, Free to not summon Blood Letters
+ Screamers - 24" turbo-boost gives you an easy flanking threat and the occasional linebreaker

Psychic Phase Control
If you're running a factory, you'll have 20+ dice.  If you're running a circus, you'll have 12+ dice.  Armies that have a lone psyker will never get a power off.  Your key units have psykers in them, making them hard to witchfire and maledict.  With continued application of these armies, your opponents will stop playing psykers.  Just as planned.  Against most opponents, I'll go straight Malefic, but if I need shooty, I can go that way too.  Because we have -

++ Tzeralds - Basic WC batteries.  Divination can give some nasty shooting with FFoT.
++ Psychic DPs - Though the best ones are in the CSM codex.
+ Other Psychic Heralds - Biomancy Slaaneshi/Tzeentch Heralds can turn monstrous. 

Flexibility
Because you can make new units, you can react to emergent situations.  The impressive toolkit of the fast attack and troops units are at our beck and call with incursion and summoning.  Psychic buffs, free portaglyphs, IC killers and chariots are available via summoning.  With a strong base of preferred heavy hitters like Flesh Hounds or Screamers, the army can specialize mid-combat. 

++ HoS with Seeker Chariot - An immediate threat.  2d6 S4 rending auto-hits followed by 6 S3 WS7 I7 Rending attacks will usually wipe a 5-man squad.
++ Screamers - Fast, Armorbane Bites, HoW
++ Daemonettes - Fast, Rending, Rending, Rending
++ Tzerald w/ Disc - Joins with Screamers, zooms across the board
+ Plague Bearers - Nearly impossible for many armies to kill.  Glance armor on 6's mean that a full summoned squad assaulting a 3 HP vehicle has a 22% of hulling it.

The Primary Weaknesses of Daemons

Armor
Can't live with it, can't, damn, we can't really take much.  So we live without it.  While daemons could care less about the anti-armor weaponry of our opponents, there is little that can menace opposing armor available.  This is compounded with enemy fliers, where the little firepower available will be lost on snap shots.

++ Melta
- Flyrants
-- Night Scythes
-- Storm Talons
-- Land Raiders

Not Winning Hard Enough
Daemonic Instability can wipe out units that fail to win assaults.  If your opponent can force multiple assaults, this can spread across much of your front line.  Only take fights you are sure to win.

++ Playing against MSU
-- Multiple Assaults
- Large Squads

Warp Storm Table
The dice gods are made literal in this baleful abomination.  The good fails to outweigh the bad, with a sure thing turned into a rout with every roll of 4(all daemons -1++).  It adds time with the most common rolls, usually failing to turn out any damage and making an already model-and-math heavy army into even more of a time sink.

++ Kairos Fateweaver
+ CAD giving a re-roll on the warlord table
+ CSM Primary, Factory Secondary
-- Just as Planned

Instant Death
Even the mightiest Daemon Prince can get insta-gibbed by a Wraithknight.  Grey Knights aren't as bad as you think - except for the force psycannon Dread Knight.  Play careful with your multi-wound models.  A power fist can ruin the days of flesh hounds and screamers.

- Force Weapons - usually dead before they swing
-- T5