Because only a small portion of the "roof" of the vehicle covers the passenger compartment (and because the V12 engine sits where most vehicles would stow a retractable roof, Lamborghini has chosen to use a two-piece carbon fiber roof that must be removed by hand and stowed in the storage space beneath the Aventador's hood.
Where the Aventador coupe displays its V12 engine beneath a large, louvered glass window, the Roadster adds a metal "spinal column" to add back the chassis rigidity lost by removing the roof. The engine is still viewable through a pair of hexagonal openings.
Azzuro Thetis, new, metallic blue color, has been added to the Aventador's range of colors--although, I prefer the matte black that the coupe is often seen wearing. A new style of wheel also debuts on the Roadster.