Direct3D 10 only allows graphics commands to be issued from a single thread (there is a multithreaded mode, but Microsoft explicitly warns against using it due to its poor performance). In an API such as Direct3D, issuing graphics commands involves a fair amount of CPU overhead. Given the trend towards increasing the number of cores on a processor rather than the performance of a single core, it is desirable to efficiently spread this work among multiple threads.
Direct3D 11 adds the ability to create display lists from multiple threads and execute them from the main rendering thread. In addition, the Device (which creates resources) has been separated from the Context (which issues graphics commands). This enables creating resources asynchronously. Deferred Contexts are used to create display lists and the Immediate Context issues graphics commands to the GPU, including the execution of display lists created on Deferred Contexts.
Unlike the other features in Direct3D 11, multithreaded rendering is not a hardware feature at all. With the appropriate drivers, D3D10 (perhaps even D3D9) hardware will be able to perform multithreaded rendering efficiently (some level of multithreaded performance will be available even without new drivers, but it was unclear what the limitations would be in this case).