Student to faculty ratio

This is one reason admission selectivity matters. Top colleges tend to be selective. They also tend to have relatively small class sizes, on average. Small classes + selective admissions = discussions that aren’t dominated by dumb (/unmotivated) students. Ideally.

That doesn’t necessarily mean they are all good discussions. Discussion may get dominated by students who aren’t stupid, but who bloviate. Imagine a young Donald Trump in your first year Greek Thought & Literature class. A good instructor will insist that he stay on topic, support his arguments with evidence, express them coherently, and respect other students. It may take an instructor with more experience than the average TA to manage a young DT. This is the kind of learning experience he evidently missed as a transfer student at Wharton.