Dark Angels Tactics: Headquarters
To kick off my unit-by-unit analysis of the DA book, I thought I'd start from the top with HQ. Dark Angels have quite different HQ options than a normal marine army, as you will notice. One big thing you should notice though Azrael: The head honcho of the Dark Angels. I want to like him more than I do, but for a 225 point price tag, I feel that you can get better elsewhere. If you really do want to use him in an army though, then I personally feel he's well off joining a unit that will get him in combat with his S6 master crafted power weapon. One good use of him is to attach him to a unit of terminators in a Land Raider Crusader, giving them his 4++ save, which diminishes the reliance the unit would have on Storm Shields and allow them to take some more Lightning Claws without worry. Alternatively, a combat oriented Veteran Squad isn't a bad choice, though it can cost a lot of points.
Belial, Master of the Deathwing: This is the DA version of a Captain in Terminator Armor. He lacks the WS 6 of vanilla commanders, as well as their 4++ save, but he's very cheap and makes your terminators into troops and is Fearless, so I feel that you always get more than you pay for. Since he only has a 5++ from his terminator armor, I always give Belial a Thunder Hammer and Storm Shield. I tend to always make space for this guy, even if I'm not running a terminator heavy army, for 130 points, he's a bargain.
Sammael, Master of the Ravenwing: An interesting special character with 2 different versions available: Jetbike mounted or in an AV14 Landspeeder. On a jetbike, I tend to fly him off on his own, picking off vulnerable targets and being a general pain in the butt for the enemy. He has Eternal Warrior, Fearless, T5, and a 3+/4++ save, which makes him a pain to get rid of, combined with his high BS5 and Plasma Cannon, he's a wonderful irritation, keep him out of combat though, unsupported having A3 with a master-crafted power sword isn't as nice as it sounds on paper. When mounted in the Land Speeder, you've got a highly armored fast skimmer that is dishing out 4 S6 shots and 3 S5 shots, all twin-linked on his BS5. Speeder Sammael seems to be the more popular option, since he is great at last minute objective contesting, though I prefer the jetbike.
Ezekial: Here is a character that I really wish was better. He comes well equipped with a 2+ save, master-crafted force weapon, Psychic hood and 12" aura of Fearlessness. Ezekial also has a special Psychic Power: Mind Worm. This power really isn't that good, it's Heavy so you can't move and use it, plus it only has a limited 18" range. If you do cast it, the model takes a LD test or dies, and this does affect Fearless models. I'll go over the other powers in the Librarian section below. In short, I think that he is interesting, but not sure if he's 50 points more interesting than a normal Librarian.
Interrogator-Chaplain: The Interrogator-Chaplain is probably my favorite HQ model in the Dark Angels book. He has the same stats as a Company Master, but comes with a power weapon and the standard Chaplain: The whole squad gains fearless and re-roll failed hits on the charge. I use one in Terminator Armor and attach it to a TH/SS squad of Deathwing for an extremely hard hitting unit charging out of a Crusader. Over a normal marine chaplain, they have +1 WS/BS/W/I/A, which is pretty sweet if you ask me.
Company Master: This guy got the short end of the stick in the Dark Angel book. He is only WS5, nothing fancy like Orbital Bombardment, can't take a bike or Terminator Armor, and can't even take a Storm Shield. For my HQ slots, I'd much rather go with Belial or an Interrogator-Chaplain. One interesting note though, they can take 3 weapons, which vanilla captains can't do; their pistol can be exchanged for a plasma pistol, their chainsword for a power weapon, power fist, lightning claw, pair of claws or a thunder hammer; and additionally they can take a stormbolter or combi-weapon... that's a lot of hands.
Chaplain: 20 Points cheaper than the Interrogator-Chaplain, -1 W/LD, and can't take terminator armor, otherwise he's identical. If you're not going terminator heavy, or are trying out a combat veteran squad, he isn't a bad choice, especially for his points. Compared to a normal marine chaplain, they have +1 WS/BS/I/A, all for 100 points, not too shabby.
Librarian: Standard force weapon wielding chap with a couple interesting powers. The first is Force Barrier: special Invul save, instead of making a normal save, you take a LD test, if you pass, you don't take the wound. It's not a terrible power, though I would like it much more if he were LD10, on LD9 though it's statistically the same as having a 2++ save, provided you aren't affected by any LD modifiers. The second power can be devastating if you are lucky, it's a template attack with S2D6-2 and APD6. Aside from the short range, this power can potentially devastate units, should you roll S8+, you're autokilling Nobz and tyranid warriors, though the chances of that are only about 1/6, otherwise every template has a 50/50 chance to ignore marine armor and average S5, so it could do some damage. The killer with this guy is that all of his powers are very short ranged, and if you use either of them, you're losing the force weapon. Still though when compared to a vanilla marine Librarian, they have +1 BS/I/A, though their powers are much more limited. Also, I feel I should point out that NO Dark Angel characters can cast 2 spells per turn, not even Ezekial.
Command Squads: One important thing to remember about Command Squads: Every Independent Character in your army allows you to take one, this includes all special characters (except Sammael). They are 5 more points than buying 5 veterans, but they also gain fearless. They only get access to 2 power weapons, but that goes to 3 with the Company Champion (only if you have a Company Master). 2 models may take special weapons as well, and you can always take an Apothecary for FNP. For every Interrogator-Chaplain in your army, you can upgrade their standard to the Sacred Standard, which makes all units within 12" fearless, and if you take Azrael, they can take the Chapter Standard which does the same, but also gives the unit +1 A per model. Also, don't forget, even though Belial has special rules for faux command squads, he still allows you to take a real command squad as well.
There you have it, a breakdown of the units that you'll find leading your Dark Angel force. Personally I use Belial plus an Interrogator-Chaplain. Both come with a lot of kit for their cost, and add quite a few special rules to take advantage of.