The tone of the poem is that of a candid thought which evokes mysteriousness of Nature’s beauty. When the poet says the woods is lovely, dark and deep, one also gets the idea of ominous layer all over the place. The tone is that of a longing and many say poet has also expressed his death wish in it.

Following figures of speech have been used in this poem.
Metaphor. It is a figure of speech which means something else apart from its literal meaning. Here the poet uses the word “miles” in the last stanza which actually means the life one lives, the whole timeline.
Personification. When something is given with human features. Horse is a personification here who feels like a human being amidst the woods and he tries to inform this to its owner by shaking its harness bell.
Consonance. It is the repetition of consonant sounds. The poem is full of it. Such as “to watch his woods”, “whose woods are these.”

The poem deals with topics through more than one layer. The deepest thematic concern of the poem remains as suggestions. Such as
a) Death. Miles to go before I sleep has always been associated to death rather than regular nightly sleep.
b) Obligation. Even the beauty of Nature can’t stop the traveller because he has promises to keep.
c) the relation between Nature and human beings. The traveller is on his way to perform his chores but comes across the woods, the Nature stuns him yet he goes away.