What is a defining characteristic that all protocells had in common?

a surrounding membrane or membrane-like structure

Explanation:

A protocell (or protobiont) is a self-organized, endogenously arranged, spherical array of lipids suggested as a step-stone in the direction of life's origin.

Protocells are thought to have enabled the reproduction of RNA and hence the exchange of genetic information at a time prior to the advent of DNA and proteins (the world RNA hypothesis).

A common characteristic common in all the protocells is the surrounding membrane through which the exchange of genetic information or proteins takes place.