The most important factor remains as always the proper balance between all the product characteristics to fit the customer's needs. 1) What is the target? A family man may need a minivan, a girl scout a jeep, and a college student a 2-seater sports car, a tradesman a workhorse pickup. 2) Then mass product average (vs. custom made) equilibrium between features and cost, size and weight, roominess and aerodynamic section, Total Cost of Ownership vs. serviceability, design and durability, etc...
Market criteria... I mean, who buys a car based on a rational decision anyway? When it comes to buying a car, most buyers will make their decision based on an emotional connection to a certain brand, to status, to whatever... probably, there are as many market criteria as there are cars on the market. Okay, that may change in the near future with fuel prices climbing and economies shrinking.