As someone currently running an RP, I would say making sure to keep a consistent sense of structure and framework to the setting while facilitating an environment that encourages player interactivity.  

In the context for a forum NRP, that to me means making sure that anything the GM does is in the name of acting as effectively a prompt generator and as the one who steers the general direction of an RP.  

When running an NRP, my tendency is to usually make factions that serve a story arc purpose other players can use as a foil for their own societies.  This method can work to pretty great effect by giving some central antagonizing force to handle or otherwise something to reference from.