Like Le Havre, La Promesse is a small french-language film about undocumented immigrants and the natives who sympathize with their plight. But while both films avoid the pitfalls of the socially conscious film - most notably the potential to become unrelentingly dark - Le Havre is slight where La Promesse is deep; abstract where La Promesse is vivid and real; carefully composed where La Promesse is messy. It's arguable whether it makes this the better film, but I have a hard time believing any viewer wouldn't be more affected by it.
It's no surprise that the Dardennes were documentary filmmakers before La promesse - the film's aesthetic is meant to evoke reality and indicate to the viewer that they are as close as they can be to living within the subjects' world. But what's much more impressive is how structurally intricate the film is, both as a character study of the young boy and an exploration of the immigrant experience. This is a reminder that "natural" almost never means simple, and often a work that feels like there is less artifice and more "authenticity" in the narrative requires a much surer and more intellectually prepared hand to guide it. I don't know if I'd say La promesse transcends its humble ambition as a work of art, but its unquestionable that there should more movies like La promesse made, and if that happened film would be in a much stronger place.