Not only the quality of the system itself, people forgot the documentation! I still use my father's TEAC AX-55 MkII amplifier: the service manual includes full PCB printouts, block diagrams, schematics, and component list with all the part numbers and description: http://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.ph...

In the rare case something fails, you can literally keep rebuilding the damn thing just like Theseus' ship. This level of documentation is just completely alien for today's consumer electronics.