Dull would he be of soul who could pass by (line 2)
The speaker's message is that you have to take the good things as they come, because they won't last for long. The dull person can't appreciate the transient nature of beauty.
This City now doth, like a garment, wear The beauty of the morning; silent, bare, (lines 4-5)
Garments are things you can put on and take off, and even throw away when they get old and ratty, or if you accidentally throw your whites in with your colors (doh!). They are transient by definition. At this moment, the city happens to be wearing a particularly stunning garment.
the smokeless air. (line 8)
There's a reason chimney sweepers appear so often in 19th century depictions of London: it was a smoky city. Not to mention the frequent fogs that appear on a chilly London morning. In other words, "smokeless air" was something to get excited about for a Londoner in Wordsworth's time.
In his first splendour (line 10)
The poem is about making the old seem new again. Even the sun is remade every morning. Each day is a new and transient world.
And all that mighty heart is lying still! (line 14)
This moment will not last long. The speaker catches London at a time after the sun has risen but before most people have awoken for work or play.