What aspects of the wind does Shelley emphasize? What is his tone in this section?

Shelley emphasizes different aspects of the wind, but in four out of five of the poem's sections, Shelley focuses on the west wind's power to cause movement and change. 

In the first part, he emphasizes the west wind as a "mover and shaker" that drives the autumn leaves to new places, ending the stanza with:

Wild Spirit, which are moving everywhere;/Destroyer and Preserver ...

This aspect of the wind as mover continues in part two, where Shelley describes the clouds as "shook [by the wind] from the tangled boughs of Heaven and Ocean," and spread by the wind like "bright hair uplifted from the head." 

In part three, he offers a moment of peace, remembering the gentler winds of the balmy Mediterranean, though even there the wind makes the waves move (he calls the waves "chasms" that "cleave" the water). He then returns to the more fearsome autumn wind in the last two parts. Shelley especially wants to be an element of nature in these last two sections, blown by the wind, which he describes as "Uncontrollable." He asks to be lifted by the wind as if a leaf or a wave or a cloud.

In the final section, Shelley plays on the word "leaves" as having a double meaning: it is both the leaves on trees and the leaves of a book. He wishes for the powerful wind to blow the leaves of his writing all over the earth: "Scatter  ... my words among mankind!" he implores the wind. Power and motion: these are what he yearns for that west wind represents.