Of course, as a competitively casual player, I find that some wargears/units are worth mentioning as they have interesting/powerful application on the field;
1) Sacred Standards: Despite their 'one-per-army' limit, they are very helpful for infantry-based armies as they provide a anchoring point for the whole army, with the Standard of Fortitude being the most amazing standard of them all in my opinion with their conference of Feel No Pain to every friendly Dark Angels within 12" of the banner. This can be used in a Dark Angels Terminator or Marine-based army. The Ravenwing I assume would be able take the banner but it appears that there is a errata over there which I have to wait to see what GW's stance on it is due to Relic Banner being nonexistent in the wargear section, but the Sacred Standard being present in the wargear section.
2) Tactical squads: Oddly enough, it harks a return to 3rd-4th edition as a 5 man squad could now have a special or heavy weapon in the squad. A particularly interesting application of this would be that you can get tons of flakk missiles for cheap with a five man squad with a flakk missile launcher is 95 points. You can also take a full squad and take a special weapon along with a Razorbacks with twin-linked assault cannons or flamers for anti-infantry. However, unlike the Vanilla Marines, they have to pay for their special weapons.
In essence, the Dark Angels army has the option to not bother taking dedicated anti-air units/flyers to gun down aircraft. Necrons will hate the proliferation of these guys along with a command squad with the Sacred Standard command squad behind an Aegis Defense Line. Add some cheap devastators with flakk missile launcher for taste.
3)Deathwing: Deathwing players can now take a full squad of 10 of them and they have the Splitfire USR. Another noteworthy rule that is specific to the termies is Vengeful Strike which allow them to have twin-linked weapons when they deepstrike during their turn. And by all weapons, they mean every weapon.
In addition, their Deathwing Assault is much more potent now. Instead of half your unit coming down on turn one, you have the option to have every Deathwing to come in during the first turn or the second turn (but this must be done secretly) and you do not have to make any reserves. In other words, they are really reliable. Combine this with teleporter homers from the Ravenwing, and you can have a dominating presence during your second turn with your shooting phase.
As noted before, this also gives the army options to not take flyers if they want to as twin-linked weaponry could help mitigate the lack of anti-air for Terminators. Also they have hatred (CSM) in addition to their Fearless special rule.
4) Ravenwing Attack Squadrons: Their Land Speeders are now able to have typhoon missile launchers which would help in providing greater long-ranged firepower. In addition they also have Hit-and-run which is amazing for them if they are forced to be stuck in close combat. However, they are no longer fearless, but instead got a new rule called Grim Resolve which is Stubborn plus the inability to automatically fail a Morale Check. This unit is interesting as everything in it is scoring.
5) Darkshrouds: They are interesting for Ravenwing armies, as they allow any friendly unit within 6" of them to gain Stealth, which would help in their defense during turboboosting and such. Sadly enough, from the wording it appears that two Darkshrouds being next to each other will not confer their 'stealth' field on each other. However they all come with Shrouded which makes this particular wording understandable, as it would potentially be broken if done right.
They are not an offensive unit, but they help in the overall defense of the army as I can just park a Darkshroud near an Aegis defense line and allow the unit/s within to have Stealth. Combine with that firebase that I talked about in 2), this could be potentially nasty.
6) Land Speeder Vengeance: I would say that their charged mode will be used more often than their burst mode, as the prospect of a large blast plasma shot is hard to ignore especially with the proliferation of infantry heavy armies these days.
7) Nephilim Jetfighter/Dark Talon: Flyers are flyers, but I feel that the aircrafts somewhat suffers from being very specialized. Surprisingly for a book with flyers, the flyers could be ignored.
The Nephilim is designed with anti-air in mind, with a secondary role in anti-vehicle and its special rule of changing 'Weapon Destroyed' results to 'Immobilized' could be amazing if it were not for the weapons choices that plagued this particular flyer. Of the two options that the Nephilim are allowed to take, I would say that the Avenger Mega bolter could be good against light vehicles/light aircraft but it is utterly useless against AV12 and above vehicles of any sort. The lascannon suffers from it being a single shot weapon (despite its twin-linked status), but honestly I would take it if all you are going to face are Stormravens and Heldrakes. The Blacksword missile also suffers from this as they are single shot weapons and they are not blast weapons, and their strength is the same as that of the Avenger Mega bolter. It could be quite ineffective when faced with certain armies, and thus caution must be taken with this.
The Dark Talon is a dedicated anti-infantry unit, but even then it is not really effective at its role and it is utterly overpriced for what it does. Its statis bomb does have some unique effects, but with the stats of a lasgun it will not be doing its job very often. The rift cannon is also not as effective.
Between the two, I would take the Nephilim.
7) HQs: Alright, I believe that I am at the last leg of my review here, and I will say that a few HQs stand out. I will now start with the Generic HQs. In addition each HQ has access to the wargear section which allows for a greater customization of your HQ depending on the needs of your army. All of the HQ gains access to Command Squads, and depending on their upgrades they can gain access to specific command squads (example would be a Terminator Librarian would gain access to a Deathwing Command Squad). However, these command squads are locked at their size capacity, so you can't add additional units to the squad (this particularly sucks for the Ravenwing command squad as they only have three members ).
As for which HQ is the most effective I would have one choice in my mind; The Librarian. He is cheap and he has access to what is perhaps the best psychic discipline in the book; Divination. With the particular emphasis of the Dark Angels in shooting, this upgrade is perhaps the most precious psychic discipline to take.
The Chaplains are interesting for assault-based armies, but in my opinion the book's main strengths are in shooting and thus people would tend to ignore them in favor of other HQs. The Company Master is essentially a Dark-angels flavoured Captain and thus suffer from the same problems as the Chaplain.
However the dark horse of the HQs in the book would be a Techmarine. The reason why this is such is due to their unique way of being incorporated into the HQ slot. As the wording goes;
For each HQ choice in your army (not including other Techmarines or Command Squads of any type) you may include a Techmarine. These selections do not use up a Force Organization slot).
This means that you can have an 'extra' HQ for each of your HQ slots, and with their rather cheap point cost they could help in many ways, especially with the right wargear. They also have the abilities of the Vanilla HQ techmarines but with Grim Resolve included.
As for the special characters, Ezekiel and Asmodai are perhaps the most subpar of the HQs, and you can get cheaper and more versatile equivalents in the librarian and chaplain respectively.
As with the old codex, Belial and Sammael are used to unlock Deathwing Terminators and Ravenwing Attack Squadrons as troops, respectively. However, there is now a third option for unlocking both types of units to become troop choices; Azrael. In this sense he could be the go to HQ for unlocking both types of units as troop choices and you do not have to spend the points on two different HQs to unlock them as troops.
Let's start with Azrael. He is perhaps one of the most potent warlords in the game as he is the only warlord in the Dark Angels book that could choose his own Dark Angels Warlord Traits instead of rolling it. However, if he chooses a Warlord trait from the Rulebook, he has to roll them randomly just like anyone else. The whole army also uses his leadership for morale, pinning or leadership tests.His armor is an artificier armor with the FNP (6+) rule which is cute in my opinion. His cost has also gone down a little which is fun, as he still keeps all his old toys from the previous edition. Unfortunately, Azrael does not have Orbital Bombardment
Belial is now more expensive, but in return he gets some buffs in his stat and also gets a precision shot on a 5+ and also any unit that is composed of models with the Inner Circle rules that he joined do not scatter when he arrives via deepstrike (could be subject to nonsense when used with non-Deathwing Inner Circle units, but even then I wonder why). His warlord trait kinda sucks and does not help the army as a whole. He still gets to switch his weapons around which would be a good thing for everyone who armed him with lightning claws or Thunderhammer/Stormshield.
Sammael is more or less the same (his stats have buffed up but besides that he is still the same) but instead, he is now an IC. Goodbye the days where he could not be attached to any squad! As for his Warlord trait, it is somewhat helpful for himself and the unit that he is attached to, but otherwise not too amazing. For these two, I would personally pick another HQ as the warlord for taking warlord traits, and let Belial and Sammael unlock Deathwing or Ravenwing attack squadrons as troop choices, respectively.
PS: Azrael, Belial and Sammael can only unlock these formations as troop choices when they are in the Primary detachment slots. So having allied units with deathwing terminators as troops are now a concept of the past.