What is something that taxicabs have that regular cars do not have?

A meter.
A meter?

Price/milage stickers all over the exterior

A TAXI sign and advertising on the roof

A funky smell

A plexiglass partition

A disco ball hanging on the mirrow

A guy with a funny accent wearing a turban

Meter unit to display fare charges to passengers. Modern meters can accept credit & debt cards for fare payments. Most have a radio to communicate with dispatch & some vehicles have security barrier between driver & passenger area.
A taxi cab driver

Normally taxis have a heavy duty cooling system with A/C cooler for driving in stop and go traffic with the A/C on.