After I finished writing the above post, I thought some more about why it is that girls object to hearing about tznius. I think it’s the idea that it sounds so negative – why should your essence be judged by something you don’t do, and davka something that is seemingly so external? I think that is more or less what you were asking, lightbrite.
I think maybe the idea is not that the be-all and end-all of your existence is being tznius. Of course there is much more to your Avodas Hashem than that. But maybe the point is that this is where it starts, not where it ends. Tznius is the outer casing that gives meaning to everything you do.
I’m not good with mashalim, but maybe it’s kind of like a police badge or uniform. It’s the thing that shows who you are and what you are about and gives meaning to everything you do.