So what makes Into the Wild remarkable is not so much its subject matter. Variations of this story play out in many lives. What makes Into the Wild so remarkable is that it focuses on a particularly archetypal telling of the story, and it does so extremely well. There is, in fact, no better telling of this kind of journey, whether literally as a journey, or in its many other guises. In fact, considering how frequent the journey takes place in the lives of many people, it is remarkable how seldom the arc of this tale is told.
What comes to mind, in cinema, to compare it to? Wim Wenders' road movies, if they were ever conclusive. A few others' work. Nothing conclusive. Into the Wild is unique.
(By the way, contrary to so much talk about the real protagonists' life, I don't see the film as a story about society, or getting back to nature, or any of that. Those are points along the journey, red-herrings or macguffins, and little more.)