G2's previews can show more than a full render can!
Since the preview system is integrated into the rendering engine, it can show the effects of individual components of the rendered surface. You can turn off specularity to concentrate on a subtle diffuse channel. Toggle the effect of shadows on and off to see if the dark corner is from a shadow or bad light placement. Luminosity, radiosity, reflections... everything can be toggled and viewed alone or in any combination.
The preview can also show raw surface properties. View an object's unshaded RGB surface color or base values of properties like diffuse or luminosity.
Lights, too, can be toggled on and off. Cycle through the lights one at a time, or turn them on and off in predefined groups. Find the light that's causing an ugly highlight, or lights which are badly placed, too dim, or simply wrong!
G2 previews can be calibrated to compensate for display and destination gamma curves. Optional industry standard sRGB and rec.709 LUT calibrations also ensure that the G2's preview displayed on your monitor matches your output video or film appearance.