I've been using DADGAD tuning since 1972. I remember on my first tour in Ireland, in 1977, how many guitar players were shocked and surprised by this sound. I came across this tuning completely by coincidence, fooling around with detuning strings and trying different sounds. It was years later that people told me the great English guitarist Davey Graham was also playing in DADGAD early on in his career. Anecdotally, Mr. Graham came to one of my shows years later at the Banbury Arts Center in the UK. I met him as he was waiting in the line for an autograph. I was humbled, and very happy that in a way he was giving me his blessings. I consider him to be the father of world music guitar.
At the beginning, I was attracted to DADGAD for the open sounds, bass in D, and the fact that I could concentrate on phrasing the melody line and just follow the bass. I was playing mainly in D position and had learned a few chords to help me sustain songs and tunes. The trick was to not always play in D and to not use open strings prominently. When I wanted to change tonality, I would use a capo, but I soon felt limited anyhow. Now, my approach has been less empiric and a wee bit more "scientific" or rational. I started to study the fretboard, harmony, chords, scales, different modes, and manners to play the same things by alternating different right and left hand techniques. I also developed harp style effects and started cross picking strings in the style of my mentor, the banjoist Bill Keith, and adapted his principles on the guitar.
In order to know the fretboard, play chords progressions, and improvise, it’s very important to study DADGAD the same way people would study standard tuning. This being said, the tuning is only a tool. The inspiration, ideas, organization and architecture of the music, feel, grove, etc. are the real keys. DADGAD has helped me articulate and structure my approach to music, but I could have probably achieved many different things in standard tuning and be just as happy. Maybe I’m saying this because I don't even notice that I'm playing in a different tuning, but my point is to dive deep into a tuning and study it hard, or you'll have no headroom for development and spontaneity. This being said, I'd like you not to think of this tuning just as an exotic departure, but as a way to think about playing anything. Indeed DADGAD is great for playing familiar tunes and having a new sound to them, but it’s also good for developing a drive/pleasure to keeping evolving as a player.