Why are favorite movie/book/tv show/etc exchanges typically low on content?

They aren't if you're friends with any film or music buffs with a solid knowledge base. When I have a chat with my more movie conscious friends about movies the conversation always includes some level of debate about acting skill, cinematography, flow of events, writing, dialogue, how well the characters mesh together etc.

If you want to successfully critique a piece of art, why not study the formal aspects of the art form some? You don't have to devote your life to it or anything, but it's good to have a repertoire of objective criteria to fall back on when explaining why you like the piece. Besides, it would make your proposed exercise less painless, more effective and you'd be able to parse hardcore reviews of films readily and thus be able to more easily discern whether there is anything out that you might like to watch.

If you can persuade a friend to learn about this art form with you, or go to some functions where you would likely meet someone with discerning taste you will probably have much more fun with this sort of thing.