Oh? You would think there is much more in common. The graphic front end is an obvious difference.
One simple example:
Avatar enters The Guild. The Guild is filled with tables and a bartender. Each table has a different board game. The PlaneShift server notifies a remote system that the avatar is within its realm. The remote system controls the artificial intellegence of the bartender. Each board could either be controlled by the remote system or two players could use a more peer to peer system for a board. Either way, PlaneShift server would monitor the virtual physics of the remote system or of the avatars. I presume you are familiar with such environment as The Guild example.
Most concurrent on-line games (or MMORPGs) implement some sort of virtual engine code to allow easy customization of the environment. What I could suggest is to support virtual engines that are external from the PlaneShift load. So, for the sake of a simple explanation, someone that knows JavaScript could control the artificial intellegence of their bartender remotely away from the PlaneShift server.
Again, I tried to keep the explanation simple.