I was able to find this.....

http://en.wikipedia.org/wiki/Classic_car

It was actually pretty interesting. Here is a part of it...

The Classic Car Club of America claims to have invented the term Classic car and thus they believe that the true definition of the term is theirs. According to the CCCA:

"A CCCA Classic is a "fine" or "distinctive" automobile, either American or foreign built, produced between 1925 and 1948. Generally, a Classic was high-priced when new and was built in limited quantities. Other factors, including engine displacement, custom coachwork and luxury accessories, such as power brakes, power clutch, and "one-shot" or automatic lubrication systems, help determine whether a car is considered to be a Classic." ......

This rather exclusive definition of a classic car is by no means universally followed, however, and this is acknowledged by the CCCA: while they still maintain the true definition of 'classic car' is theirs, they generally use terms such as CCCA Classic or the trademarked Full Classic to avoid confusion.

More common usage fundamentally equates Classic car with the definition of antique car as used by the Antique Automobile Club of America, who define an Antique car as one over 25 years old. Thus, popular usage is that any car over 25 years old can be called a 'classic car'.

25 years is generally considered a good cut-off age for such terms because it's extremely rare for a vehicle that old to still be owned or used without special consideration for its classic status - by 25 years old, a car will have exceeded its design life by some considerable margin, 10-15 years being the norm barring accidental loss.

As far as what defines it besides the year, I think all of what you mentioned. Lines, body style, drivetrain, availability and # of cars produced at the time, etc. But honestly, for you and me I think a lot is personal prefrences too. Insurance companies probably follow the stricter guidlines.