I would like to talk about the issue of the lack of an ending in sandbox roleplays, as [@Ether] mentioned.  I could see the concern that a roleplay without an end point could make things fizzle out due to there not being an end in sight.  

One solution I have seen elsewhere is to break up the roleplay into "seasons".  These chunks allow players to wrap up whatever they are doing so that there would be some sense of closure until the next "season" starts back up again.  Also, one other thing that help makes this work is that eventually there will be an overarching story arc that the GM/co-GMs cooked up.  While not everyone is required to partake in the event, the event helps tie everything together at the  end.