For years, the quality of American cars was vastly lower than the Japamese. Things have gotten a lot better, but now it's the overall design and market positioning that's really lacking, as well as a lack of cars that really target the market.
There's a line out the door for the Smart car. How hard would it be for GM (or Ford) to grab one of their little, gas-sipping city cars (like the Ford Ka), and sell it in the US? It's obvious that consumers want them, and it would give American automakers a domestic answer to the Honda Fit and Toyota Yaris.
The interior styling needs to stop being so disjointed; I have yet to sit in any American car and get the feel that people actually cared when they laid out the inside of the car. The Saturn Sky is a great example -- I can't operate the controls on the driver-side door, because they are placed right under the middle of my forearm.
It's even worse in the economy segment -- Ford and Mazda share technology, yet the interior of the Mazda 3 is such a wonderful place to be, whereas the interior of the Focus is just... dreary. The Mazda feels like a downsized BMW, and the Ford feels like... well, a Ford.