My dad wanted me to get a seat belt (actually he told me I had to get one or I wasn't allowed to drive it), and I told him that it would be unsafe on my go-kart, but it wasn't until a mate of his told him that their racing karts didn't have them, that he relented.
Safety features on my go-kart are:
Brake
Roll bar (not to be confused with roll cage)
Tacho (working within limits of engine)
Kill switch on "dashboard"
Killswitch near motor, clearly labled (in case I get knocked out/incapacitated, and a bystander needs to shut down the motor).
Gradual curves in throttle cable to reduce friction and reduce likelyhood of cable sticking throttle on (kill switch wires can fail)
Soon to be added:
Scatter shield. Cheap life saving device.