I don't mind if you put me on the spot. It keeps me on my toes.
To address your question:
This really depends a lot on the question, who is asking, if the card is well placed, say sitting comfy next to VI, or poorly placed, next to XIII.
I can say I'd never really interpret this card as authority, I'd save that for the Emperor. This is from my background in Astrology, where the Queen is ruled by the Moon, only as
the wife of the King, not if ruling in her own right, when she would be the Sun.
Melanchollic's Grace Kelly/Paris Hilton comment is pretty close to my view as well.
Obviously she can symbolize wives, girlfriends, daughters, mothers, anything feminine, pregnancy, menstruation, female disorders. There is some over-lap with Trumps II, XVII, and XVIII, all being feminine (either Lunar, or Venusian)
I interpret her as a generally positive indicator, but that might quickly change on a wimp. She is the younger, more naive side of feminine energy, compared to La Papess, who is older and wiser.
Taking it slightly into the less obvious, borrowing from Lilly and others:
When well placed, she could symbolize things that are liquid or associated with liquid, soft things, things of little flavor or substance, white things, new things, trends, fads, novelty, the popular opinion, compassion, accommodation, Mondays, the breasts, womb, belly, intestines.
When poorly placed she could represent lost objects, intoxicants, lack of courage, changeability, fickleness, moodiness, depression, miss placed sympathy, being too compassionate (bring home ALL the stray animals), superficial worldliness, pretensions, lack of a firm position, the status quo, materialism, and inconsistency (classic Moon association).
Yes, some of this could also be said of Trump XVIII, The Moon itself. And I do see them as somewhat interchangeable. They differ in that she is mundane and the moon is celestial. She is the way the Lunar/feminine energy actually manifest itself in the mundane world.
A lot of people prefer to see La Papess as Lunar, and The Empress as Venus. This works fine if the two cards are seen individually. But when you see them as part of a system including their partners the Pope and Emperor, La Papess works better as Venus, and the Empress as the Moon, in my experience.
That's about as much as I can say without a specific question to address. Feel free to throw one at me.
Ayumi