The performances that is! A good story, that gets a bit slowed down by some flashbacks. Dujardin and De France are a pair that ignites more than just a fire. And it translates to the screen. As the director was saying in the interview, the main thing was to make that relationship work and it does work.
The story with an international cast and many languages spoken (english, french, Russian ...) might feel overloaded at times, but it still works because of the actors involved in it. The fact you are involved with the characters only heightens the tension that the movie portrays. Not really that many action scenes (one fight scene in particular stands out), but the thrill of it still works.