The ParticleRenderers are responsible for the actual rendering of the particles. All they do is render a pool of particles, they do not care for any of the particles physical properties. You can see a couple of examples of ParticleRenderers in the video below:
The Rewrite comes with the default BillboardRenderer, which has the functionality from stock T3D that you already know and love. It simply renders a Billboard, with the all the same properties as in the old particle system. I.e. SortParticles, SoftnessDistance, TextureName and so on.