What is powerful about the way in which Keats writes about autumn in stanzas 1 & 2 of "To Autumn"?

The first two stanzas are powerful because of their rich, luxuriant imagery, describing autumn's bounty in ways we can see, smell and sense. Autumn is depicted or personified as a human being who "conspires" with the sun to produce a growing bounty of food. Keats, in this stanza, emphasizes autumn not as a period of loss or decline but instead focuses on the abundance  of the harvest, using powerful descriptive language: 

To bend with apples the moss’d cottage-trees, 

And fill all fruit with ripeness to the core; To swell the gourd, and plump the hazel shells 

To swell the gourd, and plump the hazel shells 

We can visualize this scene: apple trees by a cottage, moss around their trunks, their branches bending towards earth under the weight of so many apples. We can see the gourd and the hazel nuts. The stanza moves slowly, encouraging us to slow down too and dwell on the language, which builds its rhythm through devices such as alliteration or using words that start with the same letter, such as "mists," "mellow," and "maturing."

In the second stanza, Keats continues the rich descriptive imagery, here introducing smells: "the fume of poppies" and a sense that all this abundance makes Autumn drowsy, like a person who has just eaten a big feast. In this stanza we also feel the slow rhythms of Autumn, weighted down by its abundance. Now we are getting a sense of the slowing pace of the season. Autumn watches the "last oozings, hours by hours" of the cider press.

Keats loved to celebrate beauty, and does so here, capturing the slow, rich abundance of fall.