I had some correspondence with the late John McGann years ago (he was professor of mandolin at Berklee College of Music and an excellent guitar player too) about a tune he wrote called Canyon Moonrise which I really like. He was kind enough to share a load of comments and help with me - I'd contacted him totally out the blue - and I don't think he'd mind me sharing his comments.
ME:>When I first heard it I thought it was in DADGAD or an
>open tuning so I have learned a lot as well as a tune
>which I'm sure I will use elsewhere.
JMcG:>That's a nice compliment. DADGAD is great, but everything has its up and down side...I try to "emulate" DADGAD by choosing voicings that allow for a lot of ringing open strings. I've never liked the " 'ang on while I switch capo positions for the new key" mentality of DADGAD- it's great for specific keys and modes, but in others, you lose all the gains.
I had a slightly similar conversation with Denny Bartley when Last Night's Fun played in our local pub years ago. I commented on the freedom with which he played (he played in DADGAD all night I think from memory) and his comment was along the lines of 'it's as much a curse as a freedom as it brings with it its own set of limitations'. I think I had just been captivated by the sound of it at the time as a (mostly) accompanying instrument.