Wow... this is a great conversation... so many questions have been raised... one that I would like to address; the "beauty" of music... what is about one melody or another that appeals to people?
This has been questioned in what is "beautiful" when looking at the human face. A study that I read a number of years ago postulated that the more symmetrical a face, the more it is considered "beautiful", and had examples of movie stars. If you take the picture of the face of many of these people, cut it in half and mirror it, you will find that there is very little difference between that and their "normal" picture... it was concluded that we are just "hardwired" for this.
Over the ages, music has come under the same scrutiny. As mentioned, equal tempered scales don't "sound very good"; to our ear, they are dissonant and unappealing. In the middle ages, the "tritone" was considered the "devil in music"...
http://www.medieval.org/emfaq/harmony/tritone.html
It's interesting to note that so much of what we hear and is acceptable to our ears is based on geographical location. Indian music is instantly identifiable, and to many westerners sounds "odd" (I find it quite beautiful and calming)... see the following:
http://www.chandrakantha.com/articles/scales.html
Many people used to listening to "pop" music will find it very hard to "get their ears around" musicians such as Thelonious Monk. His music is a perfect example of "playing outside" the key. Westerners insist on hearing chords resolved - funny actually that this has such an impact on a listeners.
As for guitar, as you know (
), the guitar, as commonly known is impossible to tune correctly without slight modification. A guy I knew in California came up with a technique to solve this issue. Though some manufacturers use it, it is still relatively unknown:
http://www.buzzfeiten.com/index.htm
Now, back to throwing out names... along the way you "metal heads" (no, no the ones with metal plates in their heads, or the ones that wear tin-foil hats) will remember one of the lesser known but amazingly influential guitarists of the last 30 years: Uli Jon Roth. Uli was most notably the guitarist in the Scorpions and went onto a solo career. He designed many "Sky Guitars". These had as many as 42 frets!
http://www.edenwaith.com/uliroth/music/skyguitar.html
So much of this conversation is really based around the argument of "technique"... I like to think of technique this way: it is a means to an end, not an end in itself. Too many of the "shredders" out there forgot this... technique is not about how fast you can go up and down the neck, how well you kow the neck or anything like that. It's about being able to convey a musical idea to another. If you are succesful at that, you have done all you can...
I had a guitar teacher (a brilliant guitarist in California named George Cole). He once told me (as a 16 year old know it all); There are only three reasons to take the guitar out of the case: (1) To learn something (2) To have some fun (3) To make some money. If it doesn't fall into one of those, it's a waste of time. When you think about it, these same rules can be applied to many things in our lives...
In the words of the late, great FZ: "Shut Up 'N Play Yer Guitar"...
The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" but "That's funny ..."
Isaac Asimov
..and the video I've sent was just taken from and exercise video which you can't summarize the guy with. The one who knows DT knows what I say. John is on tour with Satriani and Vai and teaching at Berklee, he's one old fart not even needed to compare, my bad. 
- By Andreas Hopf