Besides the efficiency of interaction, what determines a nebula's perceived brightness?

The intensity of the illumination source, the distance to the nebula, and the concentration of the light-interacting material.

Perceived brightness is a complex characteristic influenced by several physical parameters beyond just the interaction type (like emission or scattering). A critical factor is the intensity of the light source responsible for the visibility, whether it is the nebula's own ionized gas or an external star. Secondly, distance plays a major role, as light intensity diminishes rapidly with separation. Finally, the concentration or density of the material capable of interacting with light—whether it is the gas in an emission cloud or the dust in a reflection cloud—determines how many photons are generated or redirected towards the observer.