What is the significance (or lack thereof) of Nausicaa's broken sword?
First off, I think your instinct is generally correct: This scene is meant to establish that Kushana is a very physically strong character with excellent martial skills. This becomes more relevant later, especially when more is revealed about Kushana's physical body.

There's also truth in @kumagoro's answer. Breaking the sword is a partially symbolic act, of breaking the will of the conquered.

That said, I don't agree that "There doesn't seem to be much of a reaction from Nausicaä" to the sword's destruction. Kushana drops the sword in one panel and breaks it in another; the panel between the two shows Nausicaä's reaction, which does indeed look like shock to me. We don't particularly get to see the reactions of other Valley natives, but Nausicaä's seems appropriate to me.

I think we can combine that reaction with the above concepts to formulate the following hypothesis:

Nausicaä attacks Kushana's soldiers, and Uncle Mito defuses the situation by forcing both sides to stop.

Kushana appears and is faced with a delicate situation. On the one hand, someone just killed her own soldiers, and even though that's paused for the moment, the rest of her soldiers aren't necessarily going to be satisfied if Kushana just brushes it off with no consequences. Justice needs to be seen to be done. On the other, she doesn't want her soldiers slaughtering everyone in the Valley; she needs their manpower to handle the God Warrior egg.

So, Kushana destroys the weapon used to kill her soldiers. This demonstrates her role power -- she can destroy things in the Valley at will and nobody in the Valley can stop her -- as well as her physical power to break such a strong blade. It's an act of dominance.

Nausicaä is dismayed, but to the regular people of the Valley, that sword is a tool used by royals more than a historically or culturally significant artifact, as far as it seems to me. Yes, it's a rare object, but the people of the Valley are going through a pretty darned traumatic day; Kushana destroying a royal sword is a relatively minor incident.

All that said, this scene also involves Nausicaä demonstrating a psychokinetic power that never shows up again, either, so Hayao Miyazaki may have just been making stuff up and seeing what stuck. :-)