You might want to focus on the way that the poem personifies the city of London, making it seem human. Note that we are told that London "like a garment" wears "the beauty of the morning," the river has "its own sweet will" and the houses are said to "sleep." Lastly, the entire sight is personified in the last line as being a "mighty heart":
Dear God! the very houses seem asleep;
And all that mighty heart is lying still!
If we examine all of these characteristics, what seems to amaze the speaker so much is the city's beauty and tranquility on this morning. The city throughout the poem is presented as being peaceful and beautiful, as these three lines make clear:
Never did sun more beautifully steep
In his first splendour, valley, rock, or hill;
Ne'er saw I, never felt, a calm so deep!
It is the sight of this "mighty heart lying still" in the beautiful morning sunshine that produces this sense of calm and peace in the speaker, which makes him feel more calm than he has ever felt in his life. This is a very novel perception of the city, for in Romantic literature they were normally depicted as ugly and enchaining men rather than liberating them. Wordsworth in this poem re-envisions the city, showing that it to can be a sight of natural beauty and exploring how it can bring peace to the soul.