When Nausicaa unbuttons the princess' shirt after rescuing her from the Tolmekian airship crash, what does she see that surprises and angers her?
I think this must be the Control Stone, as is made explicit in the manga the film was based on:
This stone is "a small, ornately carved sphere, in the shape of an egg. It is used to control Ohma, the God [or Giant] Warrior".
In the manga, princess Rastel (Lastelle in the film) of Pejitei hands it over to Nausicaä right before she dies.
Sidenote: The stone doesn't make its appearance in the film at all. Later on in the manga, after the events the film was based on, Nausicaä gains control over the God/Giant Warrior (in Vol. 4 of the Nausicaa of the Valley of Wind Perfect Collection, apparently).
Princess Lastelle's wounds seems to be the alternative and more common explanation. I found one more forum thread where this question was asked, and it offers that same interpretation.
This is repeated in a few more places I was able to find:
A fan-made script:
(Seeing the girl's injuries, Nausicaä realizes she is beyond help.)
The book The Anime Art of Hayao Miyazaki by Dani Cavallaro:
Illustrative examples of situations in which eyes play an especially prominent role in Nausicaä are the shots showing the protagonist's reaction to the sight of her murdered father, her apprehensive expression as Yua surprises her in her secluded laboratory, her horrified realization that Princess Lastelle cannot possibly survive her injuries, and her outrage at the scene grimply flaunting the harpooned baby Ohmu.