Earth has not anything to show more fair: (line 1)
Ah, so the speaker gives "earth" all the credit for the beauty of the scene. What about all the people who designed and built those towers and domes? Poor Christopher Wren. (Jeopardy points: Christopher Wren designed St. Paul's Cathedral in London).
like a garment, wear The beauty of the morning; (lines 4-5)
Nature brings out the beauty in the landmarks of London. Interestingly, though, the effects of the light are compared to clothing, a product of human culture. It is hard to tell nature and culture apart.
Open unto the fields, and to the sky; All bright and glittering in the smokeless air. (line 7-8)
These images give London an almost heavenly appearance, and certainly make the city seem less cramped and crowded. Nature is the vast frame that surrounds the scene on all sides.
In his first splendour, valley, rock, or hill; (line 10)
Wordsworth is famous for his poems that praise natural wilderness and pastoral life, such as "Lines Composed a Few Miles Above Tintern Abbey" (also on Shmoop). To hear him speak about his beloved valleys and hills in anything less than glowing terms is odd, to say the least.
The river glideth at his own sweet will: (line 12)
Even the images of nature play against the expectation of feeling rushed and harried by the city. The river does not allow itself to be rushed. It flows at a slow and even pace. Also, the elements of nature are like people that populate the empty-feeling city.