If you teach to the lowest common denominator in a class room it means you are targeting your least able students
"From a rational public policy perspective, satisfaction is not only a low common denominator, it is dangerous."
In the above, the individual is saying that general satisfaction with the way things are is an undemanding means of judgement. Without seeing it in greater context I think the implication is that systems or performance should always be examined with an eye to improvement.
"Unlike typical Where to Find Birds books, which tend to be written to a low common denominator, Wheatley's book incorporates the latest field reports from leading British and American experts on South American birds."
The above suggests Where to Find Birds books are written simplistically to be accessible to the general public. Wheatley's book is more academic.
It carries with it a somewhat negative connotation in most contexts but not entirely. In the example about the books, there is nothing wrong about writing bird books for the general public. It doesn't imply that the general public is stupid, just that the series is not written for experts in the field.