Is there any concept so deceptively simple as mise-en-scène? At least, as I've been revising those Jim Hillier edited volumes of Cahiers du Cinema from the 1950s and 60s, I've been noticing that a) there's no good clear explanation of the concept as it was deployed by the French critics, and certainly not from the critics themselves, and b) that I'm even a little fuzzier than I'd like to be on its meaning.
To be sure, there's a simple definition posed by David Bordwell and Kristin Thompson's introductory text, Film Art, namely, everything involving the profilmic. The advantage of this definition, undoubtedly one dovetailing with their pedagogical mission in and their tendency toward discrete categorization, is that it keeps mise-en-scène distinct from cinematography. Given this clarity and the role of Film Art in many of our initial film educations, mine included, this definition holds increasing sway over younger scholars.
The disadvantages of the Bordwell/Thompson one are several, though. Pedagogically, it's very hard to communicate to students the difference between arrangement of the profilmic with the profilmic objects themselves. I'm sure we all have stories of "mise-en-scene" analyses that simply lists props, sets, and actors. About the fifth one of these you receive, you start to blame the textbook.
In a related vein, once you settle for an "everything in front of the camera" definition, it can all become too broad and you lose sight of the aesthetic, critical and hermeneutic aspect the term should have.
Finally, there is a strong critical tradition in film studies in which mise-en-scène includes camera movement as much a part of blocking as set design. In Bazin, for instance, mise-en-scène is a practice opposed to montage. Framing, camera movement and blocking within the profilmic space all achieve the ends of montage through other means.
From what I can gather, mise-en-scène held such importance for the auteurists because under the classical studio systems, directors didn't have creative control over editing, script or even always storyboarding. Mise-en-scène was a polemic to read the authorship of the auteur in the control of the putting in place of the elements of filming. And that's something that gets lost in our post-Hollywood Renaissance notion of the auteur (i.e. Scorcese is an auteur because he does have creative control over most aspects of his work). It gets lost, too, in mise-en-scène's less prescriptive definitions.