What strikes me is the impeccable timing. In this era, comedians spent years on the road honing their craft and their timing in front of audiences. The Marx Brothers continued this tradition even after the advent of sound, testing the routines from their upcoming films in a roadshow prior to filming. Technology can be wonderful, but I believe there is no substitute for live onstage experience.
Agree with others here; the lack of intercuts make it all the more exceptional.