Cycles Design Goals
We intend to make a render engine that is interactive and easy to use, while still supporting many production features.
We do not intend to make a physically correct render engine per se, but we do think that a physically based shading system is much easier to use. With a flexible node system and shading language support, various production tricks are also possible. Not all rendering algorithms must be physically correct, but we prefer to avoid approximations where we can.
We do not intend to make a Renderman / micropolygon render engine, but we do think handling large amounts of geometry for smooth surfaces, displacement and hair is important. However we also think that rendering algorithms based on long preprocessing and baking, which are often used in such Renderman engines, are not sufficiently interactive and user friendly for most users.
As a result, we intend to improve handling of complex scenes within our architecture and we believe much progress can be made still, but not at the cost of (too much) interactivity and ease of use.