1. A system, comprising: a plurality of computing nodes, each comprising at least one processor and memory, that collectively implement a data storage service; wherein the plurality of computing nodes comprises: a front end module that provides a web services interface through which service requests are received and that is configured to parse and dispatch service requests for processing; an administrative component that is configured to allocate resources in the system, to monitor the status of the system, and to detect anomalies experienced in the system while service requests are being serviced; and a plurality of storage nodes that collectively implement a non-relational data store; wherein in response to the front end module receiving a service request to create a table on behalf of a storage service client, and wherein the service request specifies a table name and a primary key by which to partition and index items stored in the table: the front end module is configured to dispatch the service request to one of the plurality of storage nodes; in response to receiving the service request, the one of the plurality of storage nodes is configured to create a scalable table in the non-relational data store, wherein the scalable table is configured to store a plurality of items each of which comprises a value for the primary key, and wherein the scalable table does not have a pre-determined size limit; and subsequent to the scalable table being created, the administrative component is configured to programmatically cause the scalable table to be sized or partitioned in response to detecting an anomaly in the system or in response to receiving one or more service requests to store, retrieve, modify, or delete items in the scalable table.