The word order? An interesting question. First, the more obvious observations: he arranges the lines so they rhyme, and what's more, rhyme smoothly. The word order is clearly conscious, but does not seem forced. Second, he arranges the lines not just in rhythmic units but in what we might call thought units as well; each line is either a complete thought or stops at a (fairly) natural pausing point before completing the thought on the next line.
Some slightly more advanced observations:
In line two, the "or" sequence marks off a progression in both quantity and time through word order.
The first few words of several different lines clump together in striking images, often marked off by commas (such as "Bare ruined choirs").
Words are arranged for alliteration purposes ("by and by black").
The last two lines have a kind of call and response structure.