There were several things that I read about during my ramblings on the Dear Esther forum last night that I didn't mention in my previous post, and that I feel are worthy of mentioning.
1. There were numerous reports that there is some degree of randomness with which the narrations are played, both as to which ones are played, and possibly the location that triggers some of them. In other words, If you play through the game a second time, you are likely to hear some narrations that you didn't hear the first time.
2. In addition to the narrations, there are some visuals that may vary with different playings. One such scene is a dream-like sequence where you see two cars on the highway. Sometimes during this scene it was reported that you don't see the two cars, but you see a hospital bed instead.
3. There were numerous reports that during their exploring players had encountered sighting people or other creatures. Some were merely silhouettes far off on a distant horizon that disappeared if you tried to get closer, or even just turned in their direction, but some of them could be approached closely enough to make out some detail. Some of these showed some obvious other-worldly characteristics, such as hooded figures, some with glowing eyes. A lot of these reported sightings were documented with screenshots. There appears to be some degree of randomness in these sightings also.
All in all, these reports were part of a general consensus of a lot of players, that if you only play this game through one time, you're going to miss a bunch of stuff.