Source Radius and Length
Source Radius and Source Length deserve a special mention as they are what define the size of specular highlights on surfaces.
If you want to subdue this effect, check out the Min Roughness property on the light.
Min Roughness values of 0.3, 0.2, and 0.1
One last affect they have is in Lightmass when rendering lightmaps. Larger radius lights cast softer shadows (like area lights in most major 3D authoring packages). Since this is being processed by Lightmass, only Static lights will gain the effect.
The red light has a Source Radius of 0, while the green light has a Source Radius of 150; note the soft shadows the green light casts.