There's also a difference in mindset - there's an appreciation of knowledge for knowledge's sake, and there's a presumption that you can get up to speed with the basics of things quickly even if it is outside of your subject. I distinctly remember discussing calcium channels with the medical students (at an event where there was also a lot of drinking), and discussing economics with the economists.
From my first hand experience as a graduate student at MIT, a lot of the research produced at MIT generates a disproportionate amount of buzz - essentially identical work (in terms of quality) produced by a student in a "tier 3" school will not generate the same kind of recognition. The reasons for this lie in the network effects; people read and cite people who they can attest to with less effort and scrutiny.
The points regarding the instructors and student body are definitely valid and should not be ignored.
Example from my field: silicon photonics project which got a lot of press and all of them mentioned MIT, usually in the headlines. U boulder contributed to the project and got e near zero recognition.
This is simply not true. I went to what I would call a tier 3 school for undergrad and then ended up at MIT for graduate work. The difference between the two institutions was really night and day. Not only are the classes more rigorous and the instructors objectively better at MIT, but the students just think differently: they have much more ambition and are far less constrained by the status quo / what their parents did. Being around that totally changed my outlook of what is possible and what I should expect from myself, and really pushed me to make huge career and self-education strides.