That's the big question, if I had a definitive answer, I'd be a very rich man, who would have thought 30 years ago a Ford Escort Mk.1 would be worth tens of thousands of pounds!. However the longer I've been involved with classic cars, one thing has become increasingly clear, which is that nostalgia plays a very big part, and although many cars have become classics through sheer merit, there's many a car that wasn't very good, but is so fondly remembered, that it's become a classic. This is becoming ever more evident now, as some of the old Eastern block cars are starting to achieve classic status, lets face it the cars were rubbish, but there's something about them that inexplicably draws some people towards them, maybe it was their sheer simplicity.
However in most cases, vehicles become classic because they were regarded as cutting edge in some way at their initial launch, this is true in the case of the original Mini, the VW Beetle and the Fiat 500. These names have since been revived to recreate nostalgia among owners who once owned, or even just lusted after the original. One thing's for sure though, the modern day equivalent of these cars don't stand a hope in hell of becoming classic, as they just based on platforms of more mundane cars, and are trying far too hard to mimic the original charm of the first generation cars, which were the true cutting edge cars.
Other cars, such as my speciality Mercedes-Benz, have become classics because of their technical brilliance and incredible build quality. It was said in the 1970's & 1980's that a Mercedes-Benz was engineered to last a generation; something that has been proven many times over now; you don't have to drive too long until you see an old Mercedes coming the other way, whereas how many old Ford, BL or Vauxhall cars do you see? Well virtually none is the answer, although perversely many of those Ford, Vauxhall and BL cars have become classic in themselves, mainly because they are so fondly remembered and few good examples remain. My experience when driving older Fords (Escort Mk 1 & 2, Cortinas etc) is people say things like "my dad had one of those" , "didn't Reagan in The Sweeney drive one of those" , "we used to go on holiday in one of those" and "I haven't seen one of those since I was a kid"
So in short, if your dad had one, it was on a TV program, you just wanted one, even lusted after one, someone famous sang about it, or you laughed at it when they first came out then it will probably become a classic if it isn't already.
Finally, one last thought, since the use of an existing Classic; an Audi Quattro UR in the TV series Ashes to Ashes set in the 1980's, where DCI Gene Hunt coined the now immortal phrase "Fire up The Quattro" values of the aforementioned vehicle have now doubled! How fickle we all are when cars, nostalgia and celebrity mix! There's also a hint that if they ever do make another series, Gene Hunt may drive a Mercedes 190E...I don't know about anyone else, but in the last scene of the series, following a shoot-out in which the Quattro was destroyed, I spotted a 1983 Mercedes 190E Brochure on the DCI's desk!