Here are several adjectives, or more loosely, 'describing' or 'modifying' words, that best suit the themes of The Road Not Taken.
Lamented: Permanent decisions are just that - permanent. The speaker seems to be regretful that the decision can only be made once.
Individual or non-conforming: The decisions we make inform the person we will become. Here the speaker has to literally choose the road on which to travel.
Indecisive: The poem has several interpretations. Rather than a reflection on a life that "could have been," the poem, according to some critics, is a jibe at Frost's contemporary, Edward Thomas, who was infamously indecisive.
Reflective: Another interpretation of the poem is that it explores the humanity of decisions. The turmoil and struggle of life is neatly presented here as an either/or option. Life itself is, of course, much more complex than this.