A classic can be an ordinary item with just nostalgia as a "hook". All those baby boomers on the Krate bikes have money now and want to own a reminder of their childhood.

A classic can be the the fullest development of an obsolete technology, lugged steel frames, as was mentioned above.

A classic can be the first of a type, thus setting the pattern for others to follow. A Schwinn Phantom cruiser comes to mind here.

A classic can also be something that just works so well or is so elegant, it gets used for many years and is hard to improve on. An example would be a Sturmey-Archer 3-speed hub.

Dan