Background Material Evaluation
Born out of the rebellion of Deliverance, the Raven Guard are known to use stealth, assassination and highly targeted operations to cause as few casualties as possible to achieve their goals. They're keen on jet packs, drop pods, and alpha strikes, perhaps even more so than the Alpha Legion. Indeed, they are somewhat between the Alpha Legion and the Night Lords in terms of their operations. They are quick moving like both, but don't rely on fear unless necessary and certainly aren't in to the mutilation exhibited by the Night Lords. They pull apart their enemies and use infiltration tactics, but not usually for sport or with the amount of cloaked secrecy, and they certainly don't leave behind rubble and ruin, like the Alpha Legion.
Extermination paints a picture of the Raven Guard as a legion that is equal parts night and day - dark and light. Combining lightning strikes with directed target assassination (particularly against dictators and negative rulers that need to be dealt with to advance the Great Crusade), they're very effective at what they do. Yet they do not seek out the limelight. As pale as their skins and as dark as their eyeballs, they are the ghosts of the Legions when required to be so.
Legion Rules Review
Their primary rule is entitled By Wing & Talon. This is a result of how Lord Corax shaped his legion and the units contained within it. Depending on the type of unit in question, each unit gains a further unique rule. For infantry, this is infiltration plus fleet. This makes them at least as good as the Alpha Legion where required. For jump infantry and fast moving elements like bikes, as well as terminators, they gain furious assault.
These two rules combined suggest a close assault orientated army. Sure, they're no slouches in the shooting phase, but these rules enable them to both get close and to have a close combat advantage that begs to be utilised.
The second rule is Flesh over Steel. This one is a draw-back to counter-balance the above rule. In short, they cannot take more tanks than Astartes units. If anything, this reinforces the above conclusion about having to field a close combat orientated army. It is going to be one that is orientated toward getting close and using the pressure of its troops to cause the damage where required. Unlike the Alpha Legion (which is comparable in these regards), the Raven Guard of the XIX legion has a more distinct disadvantage than their brothers in the XX legion. It means that they cannot field every type of army, even if they wanted to.
For those models with lightning claws, they can be upgraded to The Raven's Talons which are super powered versions. In short, they're master crafted as well as rending. Although a little pricey, I can see them being purchased for some squad sergeants (e.g., assault squads). For some squads, upgrading every model to these talons would be an option. But it starts to get terribly expensive and unless its a death star unit, I can't advise it.
Characters in the army can be given an infra visor at a very modest cost. These are worth it when you know you're going to up against armies like the Night Lords, or in general anticipate some night fighting. Equally, they're going to be at a disadvantage against blind grenades and similar effects. They're probably worth taking on a shooting orientated HQ selection like some of the consul types.
Cameleolines can also be purchased for character models. These in general depend on the nature of the HQ in question. For many of the stealthy and infiltrating (see: By Wing & Talon, above) this can be a great boon and should be taken. But for others, perhaps not.
Rite of War
The Raven Guard's specific rite of war is the Decapitation Strike. This means the gain of preferred enemy against enemy independent characters (note: not squad sergeants, but full independent characters). They can also re roll die for deploying first and going first, as well as selecting drop pods for most types of infantry based squads short of terminators. On top of this, the potentially devastating Deathstorm drop pod can be used as an elites slot as well as heavy support.
The rite of war combination (and even without it, in fact) suggests the deployment of some back field units who watch a bunch of Deathstorms coming down (or other area-clearing units), only to be followed up with regular marines inside drop pods. All the while supported by whatever elements are creeping up the field with infiltration. The rules seem to be geared particularly to getting not only first blood, but also slay the warlord. Grab these two, and then try to enact an Alpha Strike to get the enemy done for, and the army will thank you for it. I think that if the game starts to get a bit longer though, the legion could get in to trouble due to lack of rhino-based mobility and other tanks to support their troops. Therefore, the Raven Guard player should be thinking ahead about what their tactics are going to be after the first turn or two. Do they need to take objectives, or can they go for a full-out rout and slaying of the enemy? The lack of tanks and potential mobility after the drop pods have struck are an issue that merit pre-planning.