Let the students introduce their favorite novel (or movie) to the group and let them explain why this novel is so great and what sets it apart from similar books.

I think this exercise will help with being specific, but at the cost of straying from your TRUE reasons for liking/disliking. i.e. in order to think of specific things to list, you will rationalize.

For example, maybe you really like Movie A just because you found it entertaining, but since you have to list specific reason, your brain spits back things like "dynamic character development", and "break-down of good v evil dichotomy" etc.

IIRC, there have been studies where if they just ask for people's opinions on which jam/wine/art/etc is better, the study participants are LESS accurate (less in agreement with expert opinion) when asked to name specific reasons for why the chose one over the other. I don't remember the specifics or validity of the study, though.