A lot of things have an influence on the fuel economy of a car: Driving style, weather (mileage can drop quite a bit during winter), tire pressure (we wrote about that), badly tuned engine, carrying unnecessary weight in the car, idling, cold engine & catalytic converter, etc. The driver can have an influence on many of these factors, but he won't do anything if he's not aware that he's getting sub-optimal mileage. Sadly, most people just assume that their car gets the sticker mpg, or worse, they don't even think about it (something else that a little fuel economy screen on the dash would help change).
With instant feedback – like in hybrid cars – things are different. If you see your mileage graph take a plunge after you race from a stop, accelerate unevenly or brake at the last minute without coasting, you'll think about your wallet (or see it as a game, like many hybrid drivers, and try to beat your record) and adapt your style. If you care at all about how much gas you buy, either for monetary or ecological reasons, this feedback will gradually change the way you drive and turn you into a more fuel efficient driver.
Five months ago, I wrote this: "Now there's something inexpensive [mpg-meter] that could be fitted on all cars and make a big difference. I believe that this would lead to much saner driving habits from all the well-intentioned people out there who just don't realize the impact that their driving style has on the fuel-efficiency of their cars. [...] With a MPG gauge in all cars, people would know that they are getting crappy mileage; they'd see the effect of racing from stop signs to the next red-light. A minority of people would not change their driving habits, but most people want to spend less on gas, not more."