What aspect of winter does Campbell love?

This poem, as the title suggests, is about autumn--the anticipation of the season, the experience of it, the reflection upon it and its rewards after it has passed. The descriptions to this effect are dense and beautiful--geese heeding the call of instinct in their migrations, deciduous trees abandoning their leaves to allow the cool sun to filter through the pine needles that remain. Campbell marks the season with images of the harvest and the storing of grapes for wine, providing a melodic example of the waning of summer and the waxing of winter that so readily characterizes the autumn:

Strained by the gale the olives whiten
…
And, with the vines, their branches lighten,
To brim our vats where summer lingers
In the red froth and sun-gold oil.

Campbell says in the very first verse that he loves autumn not for itself but because it heralds the coming of winter,

…the paragon of art,
That kills all forms of life and feeling
Save what is pure and will survive.

Here winter is characterized not by what falls away, but by what remains during the severity of the season, for that which endures in such conditions is “pure” and essential. It is stark to be sure, but without frill or flare or excess it is therefore the barest truth, the structure and frame of existence. By calling it “the paragon of art,” Campbell is making reference to the idea (somewhat ironically given his own florid use of description and imagery in this poem) that art at its best strips the world down to its essence, and rather than disguising its subject with flowery description, it unmasks the subject, to render it more real than life itself allows. So does winter illuminate spring, by exposing that from which it flows.

In addition, in the final verse Campbell makes a prediction:  “soon,” he says, implying in the winter months, as the speaker and whomever he is addressing sit by the fire, “the grape will redden on your fingers / through the lit crystal of the cup.” He fondly anticipates the cozy, relaxed atmosphere of wintertime, with the calming, warming nature of a fire to fight away the cold, and the satisfaction brought on by the success of the autumn harvest, represented by the wine. So although this is a poem about the autumn season, it is also an expression of Campbell's love for winter through the anticipation he feels for it during fall's approach and passing.