Something there is that doesn’t love a wall (line 1)
Doesn’t this first line sound like a riddle, or like a sentence flipped upside down? To us, this "something" sounds pretty darn mysterious and big, and the sentence construction makes us feel like this "something" lurks very near. How would the effect differ if the first line is, "there is something that doesn’t love a wall" or even, "someone doesn’t love a wall?"
That sends the frozen-ground-swell under it, And spills the upper boulders in the sun, And makes gaps even two can pass abreast. (lines 2-4)
Wow, this "something" sure is something. Here, we understand that this entity can make frost appear, and, thus, disturb the rock wall. We think it’s interesting that there are three monosyllabic verbs stacked neatly on top of each other in these lines: "that sends," "and spills," "and makes." Such verbs make us feel the momentum that they describe, and we start to think that the "something" is kind of all-powerful and cool.
Oh, just another kind of out-door game (line 21)
Because the rock wall is man-made, the business of sticking little, round stones back into the wall is the work of maintaining this artificial, man-made thing-a-ma-bob. The speaker’s nonchalant attitude toward the mending process contrasts with the more serious, ominous attitude that he possesses in the beginning of the poem when he says, "Something there is that doesn’t love a wall" (line 1).