2. It will often be miserable. This is good! Adventures are all about misery - enduring and then retrospectively enjoying. No-one wants to go off and have a lovely time all the time, right? Misery is brilliant. Walking provides it in bucketloads.
3. You are at your most vulnerable. This, too, is mostly a good thing - it will encourage people much more inclined to be kind and hospitable towards you, and will immediately break down many of the barriers of the 'rich foreigner' should you be travelling far from home.
4. You can carry everything you need on your back. The old 'tortoise effect' - your life upon your shoulders. With no more than a 15-20kg load you can carry a tent, sleeping bag, gas stove, spare clothes (even for cold weather) and all the expedition knick-knacks we tend to accumulate (notebooks, compass, penknife, map, whisky etc.) There's something deeply gratifying about being so self-sufficient. For remote journeys you can try pulling a large cart filled with worldly possessions and food/water supplies (just try making a better one that Al and I did in the Empty Quarter...)
5. You can get to places impossible to reach by any other method of transport. I'm not just talking about plane or trains or automobiles here; even my all-time favourite, the bicycle, has limitations. On foot you can scramble up a hill, over a hedge, swim across a river (maybe), through a shopping mall, into a sewer...the possibilities are endless!
It seems only fair that I also include: