The Start and Stop System is based around a special high performance starter and battery that together are able to quickly start up the engine over and over again. If you tried to replicate that with an ordinary car, you would find the starter would not have to power to instantly fire up the engine. Worse, after a half hour of starting and stopping in stop and go traffic, you would drain the battery and be stopped dead! Of course the Fiat Start and Stop System also depends on computer control to decide exactly when to stop and start the engine.
This is an excellent example of a relatively simple technology being put to work to save on gas. There are many other similar non revolutionary but rather incremental changes that could be combined to produce cars that get double the fuel economy of typical Detroit products. Fortunately for those who want to save on gas, nobody has to wait on Detroit anymore: just buy from the Europeans!