[Christ]

Apples to Apples, the gasoline engine currently in use by the prius also doesn't have regenerative braking. The electric motor stored in the housing on the back of the gasser's block is what creates that function.

Diesels have higher NOx production, yes. But there are systems in place that can work with that.

Diesels aren't inefficient at low power, they're inefficient at low LOAD... same thing with gas engines. Diesels can be run as low power as you want them, provided the engine's dynamic displacement compares to the power being created. (This is where EGR comes into play.)

At high power settings, the 1.8 Liter Prius engine uses excess EGR flow to fill the combustion chambers, which decreases dynamic displacement, keeping BSFC higher by lowering cylinder temps.

Diesels don't have difficulty handling start/stop operation as much as people like to think. Talk to a farmer some day, who runs diesel tractors.

As far as being heavy, compare the power to weight ratio (and potential power to weight) of a diesel to a gasser... they're about the same. The diesel has the potential to produce the same horsepower at the same weight (meaning smaller displacement) than a gasser.