I suppose it has to do with a combination of radical design with a continuing popularity of whatever it is. So, a Volkswagen Beetle is a design classic in the sense that it served a purpose - to provide affordable mobility to the masses - with a certain amount of style (although an appreciation of it as something aesthetically pleasing came later) and technical innovation. It was also produced for over 50 years. Another would be the anglepoise lamp, still on sale with remarkably little change over 70 years after it was introduced. Or the upright Hoover, not changed in essence since the 1920s.
I could go on and on and on, so I'll stop.
Any other suggestions?