I agree that the most common interpretation of this poem is that it suggests that taking the road less traveled is the better choice, and like post #2 said, encourages readers not to follow the crowd.
I however disagree that this is the message of the poem. Ultimately, I think the poem is simply a life commentary, and is not presenting any advice at all. It suggests that in every opportunity we have to make a decision, after making the first choice we think we might come back and see where the other choice would have taken us ("kept the first for another day"). But "as way leads on to way" it always turns out that we never go back and try the other road. There just isn't time.
I'm not sure that the speaker is lamenting bad choices or acting as a wise sage to those younger and encouraging well thought decisions. I personal see this as a simple commentary of a universal life experience: making decisions defines the road we walk in life the journey provides little to no opportunity for going back.