Lightbrite, I also wanted to point out an important difference that occurred to me between tznius and most other Mitzvos. This may be one of the reasons why it is considered so crucial, and why it is considered to epitimoze what a Bas Yisrael is about.
The fact is that the way you are dressed is both a constant and something that is clearly visible to all those whom you come into contact with. If a girl puts on a short skirt in the morning, she will be wearing a short skirt the ENTIRE day. When she davens, when she does chesed, when she goes on the bus, wherever and whatever she is doing, she is doing it while wearing her short skirt. Furthermore, this is something very noticeable to whoever comes into contact with her in the course of the day, and will probably be one of the first things they notice about her. Because, yes, a person’s chitzonius is the first things you notice about them!
Isn’t that a tremendous Chilul Hashem, that she is walking around blatantly disobeying an explicit halacha in the Shulchan Aruch for all to see?