[saurik]

The things this author likes about NaN are also properties of NULL in many environments (that NULL cannot be compared to NULL, that operating on NULL returns NULL, etc.); so while you might not see many languages default initializing things to NaN, you do see them default initializing things to NULL with similar effect.

[duaneb]

Except this is actually worse, since there are many possible values which evaluate to NaN.

EDIT: I do not know how D implements NaNs; they may have magic to make them more sane to work with.

[WalterBright]

D does not implement NaNs, it just relies on the IEEE FP hardware to do it.

What D does do is expose NaNs so the programmer can rely on their existence and use them in a straightforward manner.