What is the main message of the poem "The Road Not Taken" by Robert Frost?

A good question. Robert Frost himself complained that his much-anthologized and much-loved poem had been too deeply interrogated by critics and readers alike: he said that he wrote it originally about the indecisiveness of his friend, the poet Edward Thomas, when it came to choosing a route on their walking adventures in England before the First World War. However, very clearly the poem can be read, and has been read, as a metaphor for life, with the "traveler" representing the person moving through existence and having to decide, at one point or another, which route to take.
Frost makes clear that it is not always an obvious choice we must make when a fork appears in the road of life. Indeed, the two paths he sees are "just as fair" as each other—so, there is not any particular virtue in having gone one way over the other. If the traveler has anything on which to base his decision, it is that the road he takes "wanted wear" or had been less-used, but this is only slight—it is not a case of taking a road that is far off the beaten track, or doing something radically different; time has "worn them really about the same."
Having taken one route, the speaker considers whether he might one day come back and try the other route too but accepts that this is unlikely—he doubts "if [he] should ever come back." Taking one route, then, usually seals our fates, and the path we choose will make "all the difference" to our ultimate life journey. Even if two roads which "diverged" often do not seem to be very different at all, we will ultimately look up and remember the point at which a decision was made and realize how differently life may have turned out if we had taken the other route.