What is the theme of "To Autumn"?

Ostensibly, Keats's "To Autumn" is a paean of praise to this most inspirational of seasons. But, as is always the case with Keats, there is considerably more to this than meets the eye—a richer, more complex vision lurking beneath the opulent pleasures of nature, bursting to shine forth.
A recurring theme in Keats's odes is the fragility and transience of the natural world. And we encounter it once again here. Keats delights in providing us with lush descriptions of this "season of mists and mellow fruitfulness," while at the same time recognizing that the season, like each one of us, must one day pass, no matter how beautiful it is. But this shouldn't cause worry; new life will emerge from the old in a never-ending cycle of death and rebirth:

And full-grown lambs loud bleat from hilly bourn; 
   Hedge-crickets sing; and now with treble soft 
   The red-breast whistles from a garden-croft
      And gathering swallows twitter in the skies.

The season is drawing to a close, but nature is blossoming into full maturity as it points toward the onset of winter. The lambs are now "full grown," and the swallows are starting to gather in the skies.
 
Nature is so remarkably fruitful in all its variety. At times, it threatens to overwhelm us with the sheer scale of its fecundity. Man is the junior partner here; in his relationship to nature it is the world of the animals, the clouds, and the sweet, luscious fruit that dominates. In the midst of this endless cycle of seasonal change, there is nothing we can do but stand and admire. We must simply sit back and, in our reverie, enjoy the joyous bounties of nature, our sadness at their passing tinged with a realization that they will one day return.