The thing about poetry, though, is that it is often quite multilayered. Thinking about poetry as having "one definitive meaning" is usually a pretty shallow, narrow way of looking at it.

I agree with most of your post, but (to use your words) the section I quoted rubs me the wrong way a little bit. The fact that a piece of writing is multi-layered doesn't mean there isn't one definitive, or more than one definitive, or best or correct interpretation. You say Frost "isn't in the business of [etc]," so you must believe there's at least one definitive interpretations of his work, or aspects of his work, or this aspect of "The Road Not Taken."

A friend of mine once told me he didn't like Frost because his poems are like little wind-up-clock puzzles: very nicely crafted and complex, but once you find the key and 'unlock' it, and the machine reveals its inner workings, there's nothing left -- no depth. I strongly disagree, but I think the complaint does capture something essential about the way Frost wrote: his surgically precise verbal craft doesn't allow for the kinds of free-wheeling interpretation that one can perform on less formally controlled, rigorous poets (like Cummings, maybe?). I think my friend was saying that this precision meant Frost was without depth, because his work requires, as you put it, a "shallow, narrow way of looking at" the meaning or significance of a poem. But I don't think that that precision or narrowness deprive his work of depth or complexity or meaning. Rather, that's how Frost creates a very specific kind of depth that invites the extremely careful reader to think -- and simultaneously prevents the less careful reader from gaining any purchase in unlocking the meaning of the poem.

I don't think it's any accident that "The Road Not Taken" is universally read the way people read it. Frost knew what he was doing and what lesson people would want to take from his words. The ordinary reader, exactly like the narrator, will instinctually look for proof that their choices make a difference. Only the reflective, careful reader realizes that's not the case. So the less careful reader isn't just misled. That less careful reader is actually essential to the meaning of the poem: why do so many of us read it the way we do? What mistake are we and the narrator making? And what does that say about us?

Frost's poem "Design" more directly, and really beautifully, confronts the issue of how language and the mind produce meaning (or rather lose it and lose faith in it). In that poem a profusion of meaning and 'multi-layered' interpretations isn't a good thing. It's ominous, destabilizing, even threatening. One shouldn't never confuse a narrator with the author, but, to me, that's the essential Frost.

Aside: what piece of writing, whether it's poetry or not, isn't multi-layered? Since those layers are at least as much uncovered by the act of interpretation as they are created by it, why single out poetry for this? The fact that we think this way about poetry says more about how we read poetry than about poetry itself. And even if poetry is normally more resistant than other forms of writing to single, definitive intepretations, (again, to use your words) I don't think Frost was in the business of writing that kind of poetry.