If "To Autumn" were merely a pretty description of the season, it wouldn't be considered a great poem. In fact, to read it as a straightforward celebration is to fall into the same trap as the bees in the first stanza:
they think warm days will never cease,
For summer has o'er-brimm'd their clammy cells.
Underneath the surface richness of autumn, there is a definite sense that this beauty is transitory, and taking it for granted is dangerous.
The tone of the poem gets darker with each stanza. Even in the first stanza, there are hints that the loveliness of the season carries something disquieting within it: autumn is conspiring with the sun; the bees are mistaken to assume that flowers will always be around.
In the second stanza, there's an image of autumn sleeping: the season is found in
a half-reap'd furrow sound asleep,
Drows'd with the fume of poppies, while thy hook
Spares the next swath and all its twined flowers:
The sense here is definitely that the beauty that autumn brings is just a pause. Soon enough, autumn will wake up and continue to scythe off the rest of the furrow. We get confirmation of this in the next stanza, where the fields are stubble-plains.
The third stanza also explicitly contrasts autumn with spring; autumn's presence means that spring has passed, obviously. While autumn has its own music, what kind of music is it? Is it joyful, as one imagines the music of spring would be? We have in a wailful choir the small gnats mourn; and by the end of the poem, darkness is gathering. Hedge-crickets sing and gathering swallows twitter (when swallows gather, it's to sleep).
In other words, the poem shows us a very beautiful autumn, but alongside that beauty is an awareness that autumn is a long way after spring, and precedes the darkness of winter. The poem is about the passage of time; autumn signifies maturation, but that is only one step before death.
There's a lot going on in this poem. Thematically, it's interesting to compare it to "Ode on a Grecian Urn":
Bold Lover, never, never canst thou kiss,
Though winning near the goal yet, do not grieve;
She cannot fade, though thou hast not thy bliss,
For ever wilt thou love, and she be fair!
Ah, happy, happy boughs! that cannot shed
Your leaves, nor ever bid the Spring adieu;
In "Grecian Urn", the figures on the urn are outside time and not subject to its destructive power. So they stay young and verdant; but there is a loss there too, as they never experience the "bliss" of fulfillment. The contrast with "To Autumn" is clear. Fructification, maturation, ripeness are all wonderful things, but they imply operating within the cycle of seasons, which in turn implies eventual death.
I don't know who said this poem is "nearly perfect, but doesn't have a lot to say"; that assessment is laughably wrong.
