What is the significance of the way the poem is written? Why are there not separate stanzas? Does that relate to the metaphor of the wall at all?
Many of Frost's poems are not written in stanzas, especially those such as "Out, Out--" and "Mending Wall" in which Frost is replicating a conversational tone. I'm not sure that the form of the poem necessarily relates to the metaphor of the wall.
However, the structure of it most certainly does. The poem concerns balance. The mending wall is formed by rocks balancing on each other. On either side of the wall, the neighbors with opposing views work together to build the wall that ironically brings them together every year and keeps the apart. The neighbor represents tradition. The speaker of the poem calls him a "stone savage" who repeats the words of his ancestors: "Good fences make good neighbors." The speaker, on the other hand, declares, "Something there is that does not love a wall." Both these statements are repeated in the poem, and therefore both ways of looking at the wall are presented.
Paradoxically, both the speaker and the neighbor may be right. Good fences do make good neighbors. The fence brings neighbors together because it takes two to build it, one on each side, but it also provides a way of dividing space and establishing boundaries that people seem to need. Yet, it is more natural not to have walls. Nature seems to work against the longevity of walls; sometimes it's not clear why we need them--what we are actually walling in or walling out.