Throughout both the Aragorn and Frodo plot lines of The Two Towers, there is an increased sense of seriousness. The Fellowship of the Ring was pretty funny, with the clear contrasts between the childlike hobbits and grand folk like Aragorn and Boromir. Now, that hobbity light-heartedness is mostly gone. Instead, the language of the book has become much more poetic and serious. For example, take this passage describing the barren plains around Mordor:
They had come to the desolation that lay before Mordor: the lasting monument to the dark labour of its slaves that should endure when all their purposes were made void; a land defiled, diseased beyond all healing—unless the Great Sea should enter in and wash it with oblivion. "I feel sick," said Sam. Frodo did not speak.
For a while they stood there, like men on the edge of a sleep where nightmare lurks, holding it off, though they know that they can only come to morning through the shadows. […] The gasping pits and poisonous mounds grew hideously clear. The sun was up, walking among clouds and long flags of smoke, but even the sunlight was defiled. (4.2.87-8)
The most obvious darkness of this passage comes from the setting. Obviously, Tolkien wants to indicate to us that Mordor is not a particularly nice place. Even Sam's usual cheerfulness has been reduced to one three-word line of dialogue: "I feel sick." Beyond Tolkien's use of grim words like "defiled," "diseased," and "hideous," this passage also reminds us that Frodo, Sam, and even Gollum are going into danger. They have no choice but to travel to this awful place. These wordy passages, in which Tolkien plays up the horror of the setting, also work to build up suspense about the fate of our heroes.
Similarly, while the Aragorn plot line is less extreme, there is something so totally ominous about the ruins of Isengard and the dark forests of the Huorns. While the good side keeps winning victories over the bad, the darker tone of The Two Towers reminds us that there is still a larger war waiting for all of our buddies in The Return of the King. They have come a long way from the mostly cheerful The Fellowship of the Ring, but they still have a lot to do before they can find true peace.