The G is quite a bit larger.
The iris diaphragm on the G has smoother edges.
Camera used: Nikon D80. All shots taken in aperture priority with single-area autofocus, aiming the focus point at the same spot throughout. Occasionally the G exposed 1/3 stop longer. In all comparison photos, G is on the left and D is on the right.
Bokeh & Sharpness
Not much difference in bokeh at f/1.4, but the resolution of the D seems better.
Closing the aperture a bit, the D seems better saturated. You can also see the advantage of the G's iris diaphragm in the out-of-focus highlights.
Pretty much even at f/8.
Chromatic Aberration
Inset shows some purple fringing at f/1.4 on the D.
Purple fringing is gone by f/2.8 on the D, but the G is picking up some red fringing.
More severe red fringing on the G at f/4; none on the D.
More chromatic aberration at f/1.4 on both lenses, but different colors.
Gone by f/4 on the D; not so on the G.
Gone on both lenses by f/8.