@Jan-Nik: There are now 2 types of batch sorting in the engine - one for when there are lots of similar objects (usually dynamic objects) and one for disparate objects (usually static objects in a scene exported from a level editor). This distinction is necessary since the exported scenes usually don't have common objects, so sorting on these objects is unnecessary and very slow (I was sorting and creating lists for 200+ objects). The batching algorithm batches objects based on material and object ID (a mixture of variables including mesh, animation and frame). Then it batches objects on lighting (the engine supports more than 4 lights, it just selects an optimal subset for each object) and transformation. If there is no transformation, it tries to compile the objects into one set of buffers, so that they can be rendered with one draw call, otherwise it uses semi-instancing to render the meshes. It also caches previous draw results so that, if the data is unchanged, it just renders using the buffers that were generated previously. It's a really simple approach, but to mind, the fastest one.
Like I said, I'm not an expert, but if you have any questions, feel free to ask 
