Great acting, writing, cinematography (that shot where the camera swings open with the door and gets a shot through the spy-hole? Amazing!) and especially sound design but… [spoilers after the cut]
…what got me most pumped was the titles telling me Clancy Brown was in it. I thought it would be great to have him as a mentor character to the young cop, and then he gets killed in the second scene!? What!
Oh, the lead guy's English accent is pretty authentic, too, so that's a plus. It really grates when American actors put on an accent that only ever existed in BBC announcers in the 1930s, (James Marsters on Warehouse 13, for example) just hire a British actor, Hollywood's lousy with them! I'm glad to see that's what they've done here.
Edit: I know the actor in this case is English, I thought I'd implied as much in the last sentence ;)