Well, I just finished it, and I must say, that's a
beautifully illustrated piece of fiction. A journey, really.
At first I felt disconnected from the world, which in the beginning feels like a better HL2-level. The caves, however... those a worth paying ten bucks for.
If they made it interactive, it would have been something different entirely. As of now, it's a static world, which fits the mood of the story of Dear Esther. I somehow feel that adding puzzles would have.. cheapened the experience.
But maybe I'm wrong, and instead I would have identified with the protagonist more if there were something else to do than just walk around.