Open D is a great tuning, but I suggest you have a play about with open g (DGDGBD)

Its a lot easier to pick up if you're used to standard tuning and is great for Irish/O'Carolan stuff.

Apart from the top string most of the melody notes stay in the stame place. Its also the similar to five string banjo tuning, and much favoured by Robert Johnson, Keef, and Ry Cooder.

You can play some really nice, easy slide without too much thinking. (open, third, fifth, seventh & twelfth frets.)

Start with 'Little Red Rooster').