As far as graphics quality goes, there are six settings to adjust: texture quality, shadow quality, physics, clutter density, anti-aliasing, and a low-FX option.
With anti-aliasing enabled and textures set to the high setting, we captured an animated GIF that demonstrates the remaining settings when set to their lowest and highest options:
There’s really very little difference between the two screen shots. And frankly, the only thing you might notice when you switch back and forth is the lack of shadows at low settings.
Closer inspection reveals a difference in the clutter on the ground, though. Also, there are some items that can’t be captured in a screen shot, such as the impact of a better physics setting.
The point is that if you don’t have the graphics power to push the highest levels of fidelity in Diablo III, you’re not missing much at the game's minimum settings.