[barrkel]

It had synchronous-style code for handling animation logic, without needing hundreds of threads.

It also had some clever ideas about replicating state across the network. You want to run the simulation locally to reduce latency, but you also need it to run elsewhere to have a consistent source of truth. So some state would be calculated by the local simulation, but be updated when packets of truth arrive. Member variables could be annotated according to how they were replicated, IIRC.