There is an interesting discussion going on the Visible Evidence listserv right now about the definition of propaganda. Interesting for what individual contributors are saying but also interesting because a number of those emailing seemed to think that matter was basic and settled but the discussion showed precisely how little agreement documentary scholars had on exactly how to define propaganda.
I won't summarize the debates but in short they point to one key problem in defining propaganda. Film scholars have a set of critical priorities that lead us to avoid the term propaganda. It's value-laden, it obscures more than it reveals, and it revels in a Manichaean division between good and bad nonfiction. The problem is that the term has a wide popular usage. There is no reason scholars cannot (and should not) resist popular terminology and usage, but they can resist it only to a point. For instance, we do not need to label Thin Blue Line a propaganda film simply because it has a strong polemic and in journalistic terms is not "balanced." But by nearly every measure the Why We Fight series are propaganda films and no amount of desisting the term is going to change that. We could come up with a less value-laden term like "persuasive rhetoric film" just as we could invent a "Manichaean frontier narrative film" to replace "Western." But that's little guarantee that scholars won't substitute "propaganda" or "Western" in their mind as familiar concept-clusters. The popular term has irrevocably shaped the object study and at least some aspect of how we understand it.
At one point in his Craft of Sociology, Pierre Bourdieu notes that the scientist makes a break from common language but the social scientist has to work with common language to some extent. This is one instance that I'm fine with insisting on analytical clarity to our vocabulary but think this analysis needs to take into account the common language usage as well.