I don't know what it is, but when I finished 303 I felt like this was perhaps one of the most well-written stuff Ennis has ever done. The story's on a nameless, aged Russian colonel that starts out leading a dozen Russian Special Forces into Afghanistan to retrieve some secret documents from a downed US airplane. Interestingly, the English SAS are after it, along with the American military. It leads to some smart conversations and narratives between respectable men who seem to be in the wrong place and time of the world.
The dialogue is direct, rarely caustic or vulgarly fun like you get in classic Ennis characters; it serves for the foreboding mood created by the end of the first arc. There is a wonderful point of view on the War of Iraq and how it is to be seen historically. In fact, history is very much a strong theme hanging over the Colonel, as well as the Sargent you meet in Part two. It's built up to haunt (you'll see this in many ways, not just in apparitions) the characters, to remind them that the world seems to have outgrown them.
The art, of course, is still very much in line with the rest of Ennis' gory realism. Jacen is a cool partner to Ennis. I'd love to see more from the two, like a longer series.
I recommend this to anyone whose anyone with a brain for its universal appeal and grade-A storytelling.