1. A computer-implemented method for mediating competing resource configurations, comprising: under control of one or more computer systems configured with executable instructions, maintaining a set of reputation scores, based on performance metrics with respect to goals, corresponding to a set of replaceable control plane modules that at least partially implement at least one control plane of a virtual resource provider; receiving a plurality of resource configuration requests generated by at least a subset of the replaceable control plane modules, the plurality of resource configuration requests specifying a plurality of competing configurations for a set of virtual resources maintained by the virtual resource provider, the set of replaceable control plane modules performing management activities over the set of virtual resources; using one or more control plane mediators that are responsive to the plurality of resource configuration requests, configuring the set of virtual resources with at least one mediated configuration determined based at least in part on at least one of the plurality of competing configurations and at least a subset of the reputation scores corresponding to the subset of the replaceable control plane modules that generated the plurality of resource configuration requests; and updating the subset of reputation scores that are associated with the subset of the replaceable control plane modules based at least in part on a set of metrics that measure performance with respect to a set of user-defined goals of the plurality of resource configuration requests.