Open D tuning (also known as Vastapol, Vestapol, Sebastopol, Sevastopol and D Major) is definitely one of the 2 big tunings for a number of blues styles. Open D and Open G still duke it out for the # 1 title spot in the open tunings ring. Both are super-heavyweights. Many fine musicians play in these two almost exclusively. More blues have been written and played in them than any other tuning, with the possible exception of Standard.
However, Open D is definitely not limited to blues. Any style or genre works well.
Some say that Open D was the original "Standard" tuning for 6 string guitar, and that tablature was the original way of writing for it. I have not been able to confirm that, but it could be. Regardless, Open D is a big, rich, and full sound that can do anything musical you wish.
Open D tuning has for some players a certain advantage over Open G. The 6th string root bass note. On a personal level, I used to play in Open G a fair bit, but always found that my thumb was really longing for that bass note root on the 6th string. Truth is I just never mastered that tuning. I never find myself missing that bass note when I listen to others play.
Like so many things, it just comes down to personal preference. As an old friend of mine says ... what some swear by----others swear at!
It's good to learn some tunes in a number of tunings. You might find that you love 'em all. Perhaps certain ones will become favorites, or even just one. Richie Havens plays in Open D exclusively, and is still in love with this tuning after 47 years, (as of 2012). Pierre Bensusan plays everything from Celtic to jazz in DADGAD exclusively.
As with other open tunings containing the third, it’s very easy to make a wonderful sounding minor tuning just by lowering the string containing the third by a semitone (also called a halftone or half step). In Open D tuning (DADF#AD), lower the F# to an F. You can do this by making the open 3rd string sound the same as when you fret the 4th string at the 3rd fret.
Some cool things about four open D tunings. (Open D, Open D minor, DADGAD, and D-13 )
For one, the notes on the bottom strings --- D, A, D, remain unchanged. This allows many of your musical ideas to be used in more than one tuning.
For another, those notes are a power chord because the note relationship is 1, 5, 1 -- no 3rd.
Also, from Open D to any of the others is easy .... retuning only one string, (the 3rd or 2nd). Tuning to D-13 from Open D minor and DADGAD or back is only two strings, (2nd and 3rd).
And finally, in Open D Tuning the Minor Pentatonic scale has no sharps or flats .... D F G A C. Makes it easier to learn the notes if you wish. Not a requirement, but always a good idea.
The Minor Pentatonic is very important. It's fun to play and used so often in blues and slide that even some great players who are authorities on the blues call this scale the blues scale. However, the blues scale has a flat 5th added.
Here's how to tune to Open D from Standard tuning.
1. From Standard tuning, lower the 6th string, (the big fat one) so that when you put your finger on it at the 7th fret and play the string, it sounds the same as the open 5th string. D
2. The 5th string stays the same. Don't retune it. A
3. The 4th string stays the same. Don't retune it. D
4. Finger on the 4th string at the 4th fret. Lower the 3rd string to match that tone. F#
5. Finger on the 3rd string at the 3rd fret. Lower the 2nd string to match that tone. A
6. Finger on the 2nd string at the 5th fret. Lower the 1st string to match that tone. D
Retuning lower, the neck may relax, so notes may need tweaking a bit to sound right.