Another reason for 3d would be the seamless connection allowed between air land and sea.
In a 2d game those three everything is seperate and those three types wouldn't be close enough to be realistic.
You wouldn't even be able to look up.
Of course some genious developer could think of something new..
Another thing I was thinking was 2 additions to the three land types, water air and sea. I'd love to explore the sea and clouds, but since there is dark magic and light magic on this world then why not have heaven and hell?
They could both be somewhat chaotic places with the rules of all three land types involved. Perhaps a cliff into air that acts like sea for heaven and mainly earth and fire for hell. They would both be heavily poloarized toward light and dark respectively, with their own unique and god-related charges that cannot be found on earth. Now i'm pretty sure that DD2 will not have this but this is just me adding to what I think is a good idea for RPGs, magically charged land. Everywhere should be open to players at any level at any point in the game, that is my exploration ideal, but here I think only at the higher levels can the player survive in hell or travel in heaven. Also, only at the higher levels can the player harness the elements of either place to include them somehow in his own reprtoire for the main battle which takes place on earth.
And how about Heavenly Focus, a power granted by the gods that allows infinite focus?