Just a few highlights, including some of the styles of play (but I recommend reading the whole thing):
Some Space Marines are epic individuals who will risk all for a matter of honour, others are philosophers and tragic figures, full of passion and regret. However you choose to interpret these ideas, it is important to note that the Space Marines chosen for the Deathwatch are nuanced, complex characters.
The Emperor's Finest
Space Marines are, by their nature and design, most commonly found where the fighting is the thickest. Bred for war and trained to excel in all aspects of battle, Battle-Brothers fit easily into adventures that focus on lots of combat. These often take the form of military missions where the characters find themselves sent in to destroy targets, complete objectives, and bring glory to the Emperor by vanquishing his foes. While there are many permutations and variations on the military theme and the idea of an elite group of warriors tasked with special orders, they all share the common goal of annihilating the enemy, usually in a hail of bolter shells and plasma bolts. Military-themed games also often focus on single actions and objectives in the greater scheme of Imperial strategy.
While the Deathwatch are drawn from the Adeptus Astartes, they also work closely with the Ordo Xenos. They are an elite force within the Imperial war machine, but, they have a special place within the structure of the Inquisition dedicated to the eradication of a specific enemy of the Imperium. Games that feature the involvement of the Inquisition are likely to be more subtle and detailed than those in which the Kill-team is facing the foes of humanity in open battle. This can mean accompanying an Inquisitor and his servants into the depths of a hive world, some ancient and forgotten alien ruins, or the shadowy corridors of a space hulk, far from the support of the Imperial armies, where the Kill-team must rely upon their own skills to survive and protect their allies
Envoys, Emissaries and Assassins
The size, skills, and flexibility of a Deathwatch Kill-team mean they often find themselves in situations unique to the Adeptus Astartes. Often, at the behest of a Watch Captain or an Inquisitor, a Kill-team may be dispatched as part of an envoy to a wayward Imperial world or even an alien empire, either alone or as part of an Imperial emissary’s entourage. This can serve a number of purposes, such as making a show of force, keeping lesser Imperial servants in line, or even honouring an ally with the presence of Adeptus Astartes representatives. In this capacity, Battle-Brothers may have to use their tongues rather than their boltguns to influence proceedings.
While I have a deep love of all things Ork, I will be the first to admit that the fluff around the Space Marines has some of the best stories of the game. Being able to play out that fluff in a RPG just seems natural. I'm going to try to hold off on picking this game up (since I still need more for my Ork army), but man is it going to be tough.