An AF-S motor which will allow the lens to focus on cameras that only autofocus with AF-S lenses (D40/ 40X/ 60/ et al), and the lack of an aperture ring.
Others can offer whether the optics are better, but I have no qualms with my 1.4 D at wider apertures, and especially not at smaller apertures,
The D also sends distance information back to the camera, which is used in flash exposures. I haven’t used the G version, but I’d imagine optically both are near enough the same high quality.
In response to Ciaran’s observation, I believe the G also has the distance chip, it was just when the D was introduced it was a new feature and they needed to draw attention to it. It is my belief that all lenses later than the D models have this functionality by default.
Good recommendation to check out online reviews – bythom.com, naturfotograf offer good user opinions, photozone & slrgear statistical reviews, and flickr groups dedicated to the lens offer a good idea of the less quantitive qualities of a lens such as tonal rendition, etc