Is it acceptable to open questions, perhaps community wiki-big list, such as asking for movies dealing with genre?
To cite an example: I am unsure as to ask the question: What movies depict Cartesian philosophy besides Matrix?
The Imaginarium of Doctor Parnassus depicts the "demon hypothesis" very well, although however it does not mention in wikipedia or imdb. I was wondering if there are any such films but I do not know whether the question will be acceptable.
Questions concerning movies aren't inherently off-topic, but the question you gave as an example ("What movies depict Cartesian philosophy besides Matrix?") is not a great question as it is way too broad. Historically, questions about The Matrix have been allowed, for example, as long as it was otherwise reasonably answerable. Just make sure your "movie question" is a philosophical one; whether or not it references a movie should be tangential to the primary question. Otherwise, it probably fits best on movies.SE.
I think it is appropriate to reference films in questions and answers. Consider the work of Guy Debord where a number of films he made can be seen as equivalents of the written word.
It depends on what you mean by 'concerning' or 'about'. I take it to mean 'primarily interested in'. Therefore:
Using the movie as motivation or introduction to the concept is perfectly acceptable. But the question should be about the philosophy, not about the movie.
So your initial question "What movies depict Cartesian philosophy besides Matrix?" could reasonably be looking for expositions of a philosophical concept in popular culture, but the difficulty with the question is not that it refers to movies. That question is poor here because 'cartesian philosophy' is underspecified, and you're asking a poor question...what good does it do anybody here if I answer that question with say 'Citizen Kane' and some people upvote it and others downvote it. That's totally unhelpful to anybody.
A useful question would be mostly about a particular philosophy, with possibly some added (unnecessary) explanation of its motivation by an example from a given movie/novel/whatever.
That said, I expect most questions that mention a movie or work of fiction to be unclear and lacking in philosophical depth (like 'How about that Matrix?').
I wanted to expand on my comment a little bit in order to try to make what I consider to be an important distinction here.
"Questions about movies" is somewhat ambiguous in the context of our topicality line. The fundamental problem I would pose is the following one: does the question ask after the philosophy of film/media/etc.? In other words, does the question address itself to some theoretical or critical concern, or is the inquiry primarily about entertainment?
In practice this will end up being a fairly subtle and perhaps even subjective determination, at least up to a certain point. Your question about the Matrix strikes me as borderline but certainly salvageable -- the issue would be how to suitably enframe your concern within some theoretical vehicle.
It seems to me that a question about the Matrix could very well be on-topic if the relationship between a specific philosophical theory could be explicitly established with respect to the film in question (and of course the question is reasonably-scoped, answerable/etc.) What the community really seems to find problematic are questions that are little more than fishing expeditions.
On the other hand, if the primary concern is just trying to find movies like the Matrix, that might be a decent questions for Movies. In passing, I might note a bit more generally that, for borderline questions especially, it's perhaps not a terrible idea to think about what your primary concern really is and whether there might be a better stack for it.
That being said, I could envision some possibly on-topic movie recommendation question for films that, for instance, might foster or encourage philosophical interest and curiousity (movies which have proven effective and helpful with philosophy students of a particular level); but in my opinion it would be much better to ask questions about the writing of philosophers about film -- Deleuze writes two volumes on cinema; nearly all of Zizek's books talk extensively about film, not to mention he has made a movie (about watching movies) called A Pervert's Guide to Cinema; etc.
