I don't know what the specific red flags are, but here's a list of titles, courtesy of the Smoking Gun:
(No images, so worksafe if explicit text is not an issue)
It appears that anything x-rated with "Barnyard" or "Dog" in the title is a no-no for Canadians. Various bondage and humilitation titles also appear, but you have to wonder why "Dungeon Play #3" is admissible, but "Dungeon Play #4" is prohibited. I'm also curious to know if some Canadian censor has to actually watch these things to decide they are on the prohibited list, or do they go by the cover art?
The most vile title on there (and there are many to chose from) is "Pregnant Dog Fucker". This is a niche market I just don't even want to think about.
On a happier note, the adult room at Scarecrow Video has a third of a shelf devoted to x-rated "nunsploitation" titles. das_prompt, I'm looking at you....