The woods are "lovely," but also "dark and deep." This darkness, which seems to exert a powerful attraction on the poet, takes on a vaguely ominous tone in the light of the final line, "And miles to go before I sleep." The two concepts "darkness" and "sleep" can easily signify not only sleep in the concrete sense but also the darkness of death, in whose "deep" the poet could hide from the responsibilities of life, the "promises" he must "keep." We might thus see the poet as being temporarily spellbound by the seductive darkness under the trees, representing a release from human tasks and burdens, to which he only recalls himself by an effort of will.
The deep silence is what also holds the poet spellbound - "The only other sound's the sweep, / Of easy wind and downy flake." There is a special silence that deep snow brings with it because it acts as a cushion. Sounds are muffled when there is snow, especially if the snow is gently falling as the words "downy flake" indicate. This means that the flakes are the slow-falling, fluffy, kind of snowflake. It is as much the silence as the visual beauty that makes the rider stop.