The Whiteblaze folks count ’em out.

That thread has all the main ones covered — serenity, solitude, scenery, fellowship, communing w/nature, etc. — but here are a few others that spring to mind:

1) Learning by direct experience. Like, for instance, feeling the temperature difference when you hike above the cloud layer. You may have been told in school that air forms in layers, but it’s so much cooler to feel the effect with your own nervous system.

2) New sensations. Like having your hair stand on end when lightning strikes nearby.

3) Getting scared, and getting over it. Like having a jogger come up from behind you and scare you witless because you thought you were out there all alone.

4) Singularity. Usually happens when looking down on civilization from the trail. It’s the realization that of all those people down there who could be doing this, I’m the only one doing it right here, right now.

5) Learning from mistakes. I draw far more insight from wrong turns than I ever will from having things go as planned.

Well, that’s five for me. Additions are welcome.