Mountain bike helmets have come a long way in the last few years. An emphasis on all-mountain and freeride equipment has led to helmets with added coverage, especially on the back of the head. True downhill helmets are full-face and resemble motocross helmets. If you want to reduce weight without sacrificing safety, look at carbon fiber helmets.
As with most things cycling-related, weight and aerodynamics are huge factors in the design of a road bike helmet. Lightweight materials and aerodynamic shapes boost speed while keeping your head protected. Comfort is important too, so ventilation features like internal air channels bring in fresh air and help sweat evaporate faster.