Never did sun more beautifully steep In his first splendour, valley, rock, or hill;
The speaker returns to his bold claim from the beginning of the poem: that earth has never presented a scene quite so beautiful as this one.
Specifically, he compares the morning sunlight falling on the city to the sunlight that might cover more remote parts of the countryside, such as a valley, a boulder or mountainous cliff ("rock"), or a hillside.
These sights would have been more familiar to Wordsworth than the scenery of London, who spent most of his life in rural parts of England, such as the picturesque Lake District in the northwest part of the country.
"First splendour" just means morning.
Basically, he's ragging on his hometown, saying even it can't compare with this view of London.
The word "steep" means to submerge or cover – think of how you let a tea bag "steep" in water.
Ne'er saw I, never felt, a calm so deep! The river glideth at his own sweet will:
The speaker continues on the topic of the Greatest Scene Ever. He describes how the vision of London makes him feel calm, which is perhaps surprising because London is a huge, bustling city. That's a little like saying you go to Manhattan to get away from it all.
The speaker seems to again compare London to places that you would normally think of as calming, like the hills and valleys from line 10.
This section of the poem engages in the personification of various elements of the picture. Here the river is described as a patient person who takes his time and doesn't allow himself to be rushed. He moves according to "his own sweet will."
The river Thames is not a fast-moving river.
Dear God! the very houses seem asleep; And all that mighty heart is lying still!
You would think the speaker couldn't possibly get more excited about this view after declaring it the most beautiful thing on earth, but no: he gets more excited.
He cries out to God as if he has just recognized something astonishing he had not noticed before.
He personifies the houses as asleep, when it's actually the people inside the houses who are sleeping at this early hour.
The city looks like one big, peaceful, sleeping body. Shh...don't wake it.
The "heart" of this body is "lying still" for the moment before the city awakens for a new day. The heart probably doesn't refer to anything specific, but rather the city's energy or vitality.
The last two lines mark a shift in tone with their two exclamation marks. The tone goes from amazed to Really Amazed!