I'm always amused by Legolas and Gimli's exchange at the beginning of this chapter. Gimli says, "When Aragorn comes into his own," and Legolas replies with "If Aragorn comes into his own" (p. 854, emphasis added). It's like a little extra insight into their characters, Gimli confident and charging ahead, Legolas more cautious.
There's a theme in this later section of the trilogy of, to put it Hamlet-ishly, being hoisted with one's own petard. It comes up most pointedly now, when Aragorn refers to the Army of the Dead felling so many Mordor troops. He says, "[w]ith its own weapons was it worsted!" (p. 858). I don't have anything particularly to add to that, just thought I'd mention it. Sauron's own weapons get used against him several times, don't they? Most obviously, the ring, of course.
"Pale swords were drawn; but I know not whether their blades would still bite, for the Dead needed no longer any weapon but fear" (p. 858).
"Follow what may, great deeds are not lessened in worth" (p. 859).
"We come now to the very brink, where hope and despair are akin. To waver is to fall" (p. 862).
Gandalf returns to a theme he stated way back in chapter two of The Fellowship of the Ring, and which Galadriel reiterated: "Yet it is not our part to master all the tides of the world, but to do what is in us for the succour of those years wherein we are set, uprooting the evil in the fields that we know, so that those who live after may have clean earth to till. What weather they shall have is not ours to rule" (p. 861). Why do you think Tolkien emphasizes this repeatedly? Why might he have found that theme important?