Diction is the style in which someone speaks. It can be formal or informal, clear or mumbled. In writing, it is used to set the tone of a piece or a character. In this poem, Wordsworth is sad because he sees people in the world obsessed with buying and spending while at the same time forgetting the beauty and power of nature. The tone is mournful and regretful. The diction of the first half sounds fairly straightforward at the beginning of the poem, yet it increases in formality so that, by the poem's end, Wordsworth is using much more literary words (like "pagan" and "creed") and concludes by making highfaluting references to ancient classical myths. I would characterize the diction as mixed--on the one hand, he is upset (which betrays a more common or low diction) and yet when he reflects on the romantic past his mood rises (which betrays a more idealistic or higher diction).