Mercy again comes up as a theme, though mercy of a less surprising nature. The Rohirrim are by and large people of heroes in this story, and so they show mercy on the attackers from Dunland who Saruman had raised against Rohan.
Gandalf hasn't really been in a place to know fully what the ents (and our troublesome hobbits) have gotten themselves up to... but he knows a thing or two about ents... and hobbits. I'm pretty sure he knows exactly what became of "the miserable Orcs," but that sort of thing is probably best not disclosed while traveling with nervous companions. The malice of the forest is not subtle, even Gimli picks up on it (that or he rolled really high on a perception check). As a wood-elf, Legolas has a bit more insight, about the specificity of the wood's hate and of their non-local origin.
The discussion of dwarven and elven aesthetics stands out to me as a study in contrasts and values. What one finds wondrous and precious, the other views with trepidation and unease. Gandalf brings more to the table, pointing out that the war they fight is not just for men, elves, and dwarf, but for beings and lives outside of the bounds of society as well.
The descriptions of Isengard serve well to capture scale and magnitude. Saruman's great tower, Orthanc, is a citadel, with Isengard itself surrounding it creating a stronghold that contains a city in itself.
And then, of course, we meet (again) the cheeky wardens of the captured citadel... Meriadoc Brandybuck and Peregrin Took. Gandalf seems less surprised at their presence, which does make me wonder at his leaving Gimli, Legolas, and Aragorn in the dark regarding these two hobbits. But the surprise was a good one, and I for one believe that they earned their repast.
In the film of course, we've already witnessed the Entmoot and the decision to take on Isengard, but in the book we need to wait for a recounting. So I'll try to handle all of that in next chapter, but it's worth bringing up that Jackson did some nice balancing of pace and tone (even if not chronologically balanced) by contrasting the entmoot with the battle of Helm's Deep.
The company that rides forth from Helm's Deep to Isengard is but a hero party of named characters, not a full armed contingent expecting battle. And again, Jackson has shuffled the end of one book into the beginning of the next movie. Gandalf here seems more cautious and less self-assured, as they ride through the woods. Merry and Pippin are... well... very much themselves. No one does sassy quite like hobbits.
Most of the mystery and atmosphere has been excised, leaving some gorgeous scenery and giving all the tension to the battle scenes. I'm guessing a discussion of aesthetics wasn't considered titillating enough for the screen against all the action.