Why is "Jerusalem (And did those feet in ancient time)" by William Blake such a powerful poem?

The question presupposes that this is a powerful poem, and that, of course, is for the reader to decide, but "And did those feet," also known as "Jerusalem," has long been considered a great poem. 

Part of its power lies in its simple language.While the word "countenance" which means face or facial expression is not common to us, it was very common in Blake's time, and then, as now, all the other language is...

The question presupposes that this is a powerful poem, and that, of course, is for the reader to decide, but "And did those feet," also known as "Jerusalem," has long been considered a great poem. 

Part of its power lies in its simple language. While the word "countenance" which means face or facial expression is not common to us, it was very common in Blake's time, and then, as now, all the other language is basic: words such as "mountains green," "pleasant pastures" and "clouded hills" are easily understood.

Much of the poem's power lies in its juxtaposition of beautiful images of the English countryside with the sudden insertion of the words "dark Satanic mills," a phrase which has become famous since Blake wrote it. At first, the reader is thinking about green pastures and Jerusalem and Jesus strolling among the beautiful hills of England as "the lamb of God" (an old folktale called an apocryphal story had it that Jesus visited England). It is then jarring to suddenly have "Satan" inserted in the picture. This would remind people that at the time the poem was written (1804-08), England was rapidly moving from a rural, pastoral culture into industrialism, symbolized by factories or mills that, at that time, were uncontrolled by any laws, so that workers, even very young children, were often cruelly used and the products of the mills were suspected of being of inferior quality to handmade items.

In the last two stanzas, the poem also powerfully prophecies that a Jesus of the second coming will bring justice to England through violence. He says "bring my spear ... bring me my chariots of fire," (chariots of fire has also become a famous phrase) and in the final stanza states

Nor shall my sword sleep in my hand:

Till we have built Jerusalem,

In Englands green & pleasant Land.

Using simple language and Biblical images, such as "Jerusalem," or "chariots of fire", the poem builds a powerful sense of unease about the difference between how God would like things to be, how they are, and what the consequences might be.