There are few arguable alternatives to the concensus that this poem is a middle aged man's highly personal contemplation on death.
The tone of the poem is easily identifiable. The images of cold, quiet snow falling in a deep, dark wood temper feelings of gloom with a sort of passive calmness. The rider (which one could very likely equate with the poet himself) has stopped at the edge of the wood of his own volition, to observe the scene, and is indeed almost drawn to it.
In the second and third stanzas, the rider's horse fidgets nervously. This simple detail suggests life's gentle push to keep on going, making no room for impractical (or dangerous) stops along the way. This creates a contrast before which the woods appear to represent the cold sleepiness of death. The "sweep of easy wind // and downy flake" reinforce the muffled atmosphere.
The final stanza brings the poem's principle metaphor most glaringly to light, however. The equation of life to a physical journey with death is almost too obvious to mention, as is death to a restful sleep in the last two lines. The repetition of these two lines in conjunction with the rider's thought of "promises to keep" clearly indicates the poet's weariness. At this moment, with a long road still ahead of him, the dark glade seems to offer a dangerously tempting rest, from which he would not have to wake.
Altogether a very existential ending for a late Victorian Age poet like Frost. Although, life's struggles seem to present a meaningless, tiring effort, the poet resolves to continue on.
For a suitable musical backdrop to the reading of this poem, I suggest Soft Return, by Labradford.
"...it's clear you'll never stop going 'round..."