I see that GI and ambient occlusion are generally low frequency in space, but don't reflections and refractions require higher fidelity?
uh, I'm thinking mirror-like reflections, but there are softer reflections, like off a matt red wall (seems GI-like).
That said, just rendering mirror surfaces or refractions doesn't require more than an extra ray per interaction (one in the reflection direction and one in the refraction direction). You can add a texture and get bumpy reflectons or refractions as well.
The difference between these easy effects and the high spatial frequency caustic effects is indirect versus direct lighting. It's very easy to ray trace a a simple transparent model but a bright spot on the floor (e.g. [0]) below a reflective objects requires a lot of ray samples to capture directly.
[0] http://news.povray.org/povray.binaries.images/attachment/%3C...