I did a little research in Meta and am surprised this question hasn't come up (unless is lies dormant and unfound by me). Is ok to ask the specific question "Whats the difference between the book/comic and the movie/show for X?"
They are allowed, but only in somewhat controlled situations. This area more than most can lead to either huge list questions, or bashing in the form of a question, both of which are not allowed. If you can ask questions that don't fall into one of those two categories, then feel free to ask away.
I've asked a few of those. I would say they are on topic.
To expand on Pearsonartphoto's answer, I'd suggest limiting it to specific areas of interest. Rather than asking for a general list of differences, either ask a question that can be answered yes/no with reasoning like Is the movie rendition of the Golden Compass significantly different from the book? or ask about a given character, plot point, device, etc. How True to Canon Was Denethor's Movie Characterization? is a great example of the latter.
I think a question like What plot points are missing in the Harry Potter movies that are in the novels? is far too broad despite the fact that it elicited a long answer. I'd argue that that's not a particularly answerable question as we don't know that that answer is actually conclusive by any means.