Tattie Bogle wrote:
"And I agree that the mandolin, in standard tuning, is the most logical instrument I've encountered/tried yet (except maybe the piano!)"

I just want to break another lance for the beauty and logic of standard tuning on the guitar:

It wouldn't work well to tune the guitar in fifths because of the scale length and the consequent distance between the frets in the lower positions. I think most people would agree that it's rather convenient to have the two outer strings both tuned to E (two octaves apart). This would be impossible if the guitar was tuned in fifths. Starting at low E, this would be E, B, F#, C#, G#, D#.

If, on the other hand, we said, "tune 'em all in fourths", we would have E, A, D, G, C, F. Not too bad, but if we want to "come out even", we need a 3rd in there somewhere. If it's between the 4th and 3rd string, we have E, A, D, F#, B, E, which is the Renaissance tuning. If it's between the 3rd and the 2nd string, we have E, A, D, G, B, E, standard tuning.

It would certainly be easier if the intervals between adjacent strings were all fourths (ascending), but it's impossible, if the top string is to be two octaves above the bottom string. This would require twelve strings and probably not be very conducive to playing:

E A D G C F Bb Eb Ab Db Gb Cb (== B) Fb (== E)

Major thirds probably wouldn't work well either, although that would make it possible to play the chromatic scale without shifts.

E, G#, C (== B#), E, G#, C. Hmmmm. That doesn't look good to me at all.

It is really quite remarkable that so many complex chords can be played reasonably easily using standard tuning; all kinds of seventh chords with alterations. And then there are a fair number of other tunings that are in common use.