What is the simile in "The Road Not Taken" by Robert Frost?

I think that you hit on a strong point in the poem.  If I may offer one note of clarification, it would be that the poem is rooted in comparative language.  I believe that it is a metaphor being used, and not a simile.  The metaphor used in the poem is the divergent road, and the idea that the speaker must choose one of the two paths.  The metaphor of the fork in the road is a compelling one because it highlights the power and the agony of choice.  The speaker is poised between two equally desirable, but ultimately incompatible courses of action and a choice must be made.  There is little negotiation in this paradigm, and the metaphor of the fork in the road highlights this.  The metaphor compares the two paths to the many different chocies one faces in life.  The paths chosen in both the speaker's predicaments, and in our own, "make all the difference" in identity formation.  The metaphor hopes to highlight this.