I would love to know exactly how John in Brisbane tunes his guitar, so I can try it for myself.

I had the impression that the piano, since the time of J. S. Bach, is tuned in "well tempered tuning" and that the guitar is tuned in "equal tempered tuning", which puts them out of sorts with each other. Is that true? Why not tune/build the guitar for well tempered tuning? Isn't that what the Buzz Feiten tuning system is all about?

Rick Fielding had the right idea, in his posting, above. I know that, on the guitar, the same song, played in different keys, without a capo, will have a remarkably different character and feel. I have two versions of "Mr. Tambourine Man", on a Fred Sokolow fingerstyle instructional video, one played in A and the other in D, and they really do sound different. I also have a Stefan Grossman video (GW901), in which he demonstrates the different sound of different keys, by playing songs, in different keys, one after another. If you listen closely, you can hear the different characters of the keys, C, G, D, A, E and F. One key sounds open and light while another sounds dark and another sounds tight (especially F). It's hard to find the right words to describe the differences; you have to hear it.

I think that, on the guitar, it has to do with the different chord shapes you use, to play the same relative chords (I, IV, V, Etc.), in different keys. A "C" chord, in the key of C, has a different shape and sound from a "D" chord, in the key of D. Maybe it has to do with chord inversions and open strings, but different chord shapes have different "voices". Many musicians use this feature, when they play duets together. They will each play the same chords, but in different shapes, achieving a mix of two different "voices" and a heightened interest from the tension thus created.