Robert Frost is a name which can't be separated from American literature. No, even more than that: he belongs to world literature. Why is that? Well, for several reasons. First, his poems are timeless. They seem to have been written in a time long gone. But that is not true. Frost is a person of the first half of the twentieth century, and while his contemporaries wrote poems as inscrutable and dense as London fog, he stuck to the old way of writing. Yes, he uses rhymes, and yes, he writes about nature, as did Wordsworth and Blake. Still, nature was, is, and always will be a topic, especially in the twenty-first century.
Secondly, Frost's symbolic meaning behind his - apparently simple - poetry is striking. Take, for example, the poem "The Road not Taken". Everyone of us will come to the point in our life when we have to face existential decisions. And voilà, there are the roads one (or some) of which can't be taken - and some might say: "I took the one less traveled by / And that has made all the difference".
Thirdly, the psychology of it. Similar to Browning, Frost can slip inside other people's heads as easily as one can count to three. He tells the tales of simple men and certain encounters - makes fictitious people alive and shows a greater truth by relating the stories of their experiences. There is, for instance, this wanderer who admires a certain mountain from afar. One day he meets a simple farmer who lives at the mountains's foot. The wanderer is sort of shocked when he gets to know that the farmer has never been on the mountain top, even more so, when he is told about a miraculous spring that can be found there. The farmer is simply not interested - you see, there are two philosophies which, as it were, hit upon each other.
Fourthly, and lastly, the internationality of Frost's poetry has to be mentioned. As an American who lived some time in England, he knows the best of both worlds. English rurality meets New English country charm. Frost's international success is put into words by Robert Graves: Frost is "[t]he first American who could honestly be reckoned a master-poet by world standards." I'd like to point out that I am a German and I am always deeply touched when I read one of Frost's poems.
Still not convinced? Oh, but I have to stop now. "[...] I have promises to keep / And miles to go before I sleep / And miles to go before I sleep".