Possible Additions to a Ray Tracer

TEXTURE MAPPING:
Makes those objects much more interesting

ARTISTIC SHADING:
Line Drawing, Cool to Warm Shading
Basicially designed for making things look good but NOT be photo realistic

ACCELERATION STRUCTURES: (Sub-Linear Ray-Object Intersection)
These provide a means of having LOTS of shapes in your scene without it being too slow.

MESHES:
Just a bunch of triangles in a list. (You'll want an acceleration structure however)

SHADOWS:
When doing your shade calculation you shoot a ray from the hit point to the light source. If it intersects anything then you only use the ambient light term. Otherwise you proceed normally.

METAL:
When doing your shade calculation you shoot a ray in the reflected direction. This combined with a phong component is the color you will modify the color of the object by.

GLASS:
Similar except that you will ahve a ray that goes into the object and one that bounces off of the object. (each with a different strength)

INSTANCING:
With ray tracing you can have multiple instances of the same object that can be transformed to different parts of the scene. Much easier to build scenes and much more efficient.

MONTE CARLO: (Distributed Ray Tracing)
Instead of shooting a single ray through a pixel you can shoot many rays though a pixel and average their color values.
There are a number of ways to decides where these rays go, but the easiest is to just treat that pixel as a subimage and sample it like your main canvas.
This allows for many effects. INCLUDING:
ANTIALIASING
SOFT SHADOWS
DEPTH OF FIELD
GLOSSY REFLECTION
MOTION BLUR
PARTICIPATING MEDIUM (fog)
SUBSURFACE SCATTERING (flesh, wax)