1. Bring your gods on stage and have them appear even thinner and less impressive than anything else in the story. Lack of vivid description is bad. Having them appear stupid is bad; having them spout profundities like a twelve-year-old's profound insights is worse. Even if they have a single, straightforward sphere of influence, they will not be simple.
They run your world. If they're thin, the world is thin.
I have noticed that stories with minor gods -- the local god of a spring or the evil demon summoned up by the deranged cult -- often feature them as more impressive than stories with major gods feature them.
2. A religion whose theology consists of Oppressing the Innocent.
(Boy, I surprised everyone with those two, didn't I?)
3. No landscape. No scenery. Nothing to smell, or hear, or touch.
4. Telling me the world is in danger in the first chapter. Not only does that give you no room to build up tension, it also means that the world looks like cardboard, because I haven't had a chance to see it yet. All I know is that it's in danger. Which makes it look flimsy like cardboard.
As a rule, don't tell me the world is in danger until half way through the (first) book. Start out with some other problem and use it to show me around before the problem builds to the real scale.
I have stopped reading books for this one.
5. Too schematic races. This can apply to human races as well as to elves, etc. Particularly if there is no characterization beyond the race.
6. Too schematic magic. Neatly defined, does not mesh and interweave -- with what each area can do neatly mapped out. Particularly if the wizards/magicians/sorcerers are all neatly distinct types.
7. Un-magical-sounding terminology. If your magical jargon sounds SFnal, it doesn't sound magical.
8. Magic that doesn't harmonize with the rest of the world. The woman with magical herbs and healing in the middle of a culture where life is nasty, brutal, solitary, and short. The magic in which color is immensely important and no one worries about except for magic.
These are some of the top ones for me. What are your cardboard detectors?