In the second verse, the text here is "Make straight, make plain the way." The phrases "make straight" and "make plain" are each sung to notes of the same pitch, so even musically, there's a representation of this levelness.
In the fourth verse, the text is "From stall and manger low." "Low" is sung to the lowest pitch (G), giving something of a sense of its meaning.
The seventh musical phrase:
In the second verse, the text here is "The heights of pride abasing." This line exhibits the opposite sort of feature as above: "heights" is sung to the highest pitch in the phrase (C), illustrating its meaning (albeit more metaphorically).
The eighth musical phrase:
In the first verse, the text here is "To all the world is shown" (the subject is "The promised Lord from heaven" in the preceding line). The phrase "all the world" is sung to notes of all different pitches (C A G), lending a sense of the breadth or entirety of "all."