Do you have a criterion for what makes a car a 'classic'? For me, it has to have round headlights. (Except Jeeps.)

Depending on the decade in which it was first manufactured, fins. 

Or standard (manual) transmission.
Or being in mint condition.
Or having 51% or more original parts.
Or not having been upgraded with more modern features, such as its sound system or its upholstery, or metal has been replaced by plastics, etc.
Or very few restored models are still in existence, can still be found.
Or it’s listed in classics databases as a model that fits the bill. 
~

Must have running boards, wheel spats and chrome.

Manufactured before 1980, preferably in Europe, with a price tag at least twice the median annual wage.

Mainly age, condition and originality; also rarity.

To some extent too, individuality. This is becoming itself less and less, as so many cars and vans now are merely churned out from the same few factories, and makes and models differ only in badges and minor details.