crewmeal: the shape of the Comet's windows enabled the cracks to grow faster, but it was NOT the actual cause of the cracks! de H management did not fully trust the new Redux process and insisted on adding extra rivet plates as used in non-pressurised airliners and it was these "reinforcements" which ironically actually caused the cracks.
Other pressurised airliners like the Sud Aviation Caravelle (triangular windows) and Lockheed L1011 (square) did not have similar problems as they, like all modern airliners, used only Redux or a similar process. They dol have rounded corners, as did the early Comets; later models used elliptical windows.
mad_jock: a good composite fuselage can be capable of greater frame gaps!
Unpressurised aircraft, like the DC3 Viewmaster have greater freedom of window shape and size.