About

Drones may be aerial, terrestrial, aquatic, subaquatic, and space. Safety involves awareness of how these can be dangerous.

Safety

Propellers can shred you. Depending on their size, they can be lethal. Safety measures could be guards.
Falling hazard - if the aerial vehicle falls on you, it can kill you.
In reconnaisance applications, remember that location of your drone can be detected: thermally, acoustically, with laser, echolocation, radar, with EM. Essentially - think of any wavelength on the EM spectrum, and any wave can be detected. [1]. For forces - any particle forces and motions can be detected (matter (gas, liquid, solids), air waves (acoustic), noise, water waves (sonar)).
Low pressure and lack of oxygen is an issue at high altitudes.

Heavy drones are dangerous if they unintentionally run you over, crush you, etc. Geofencing and sensors can be measures for safety, as well as readily accessible kill switches.

Water tightness is a requirement on water and below water. For underwater, combustion vehicles must carry their own oxidizer supply.
High pressure is an issue under water