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.

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.