"To Autumn" doesn't sound very much like normal speech. It has a formal quality appropriate to the ode form. It sounds like Poetry with a capital "P." Not to say that the elevated tone feels unnatural at all. It's as if the speaker were delivering a complicated three-part argument to a crowd of skeptical people who are thinking, "Who is this 'autumn' we keep hearing about? We want spring!"
You can see what we mean when we say it's not like normal speech if you imagine someone walking up to you and starting a conversation, "Season of mists and mellow fruitfulness!" Even if our name were "autumn" this would be a confusing opener. He starts right off with description and does not let up until the end of the poem. Look, for example, at the number of adjectives. There's hardly a noun in the poem without an adjective attached: "twined flowers," "winnowing wind," "soft-dying day," "rosy hue," even, oxymoronically, "full-grown lambs."
The complicated rhyme scheme contributes to a sense of formal difficulty. Some of the rhymes are right next to each other (in the first stanza, "brook" and "look"), while others are spread out over several lines ("trees" and "bees"). Finally, if you've read any Shakespeare, you'll notice that Keats sounds remarkably similar. Keats obviously read a lot of Shakespeare, and this poem has the grandeur and the descriptive range of a great soliloquy. Like Keats, Shakespeare had a thing for adjectives.