Steven Taylor, prompted by the Invisible Adjunct, observes that the appropriateness of lecturing versus using a seminar format is largely determined by three factors:

1) the level of the course (e.g., intro, advanced undergrad, grad), 2) the subject matter, and 3) the size of the class.

I’d broadly agree with Steven. All three of these factors are highly correlated; relatively straightforward subject matter is generally taught at an introductory level in large classes, while more complex subject matter usually involves small classes taught at an advanced level. The exceptions—notably the “honors seminar” version of intro—work because there’s an underlying assumption that the students in the course are already familiar with most of the material that they would have learned in the non-honors course, thus the course is no longer a true “intro” course.