Memory usage imbalance has been consistently observed in many virtualized Clouds and production datacenters. Such temporal memory utilization variance is a major root cause for excessive paging and thrashing on virtual servers even though there are sufficient idle memory on the same node or in the Cloud cluster. Memory disaggregation is an emerging research and development endeavor towards addressing these memory usage imbalance problems. This paper first defines and characterizes the concept of memory disaggregation, and discusses the demands and challenges of efficient memory disaggregation in cloud datacenters. It then examines some promising research issues, design choices and directions to overcome some of the challenges posed by memory disaggregation. Specifically, it proposes two major new research challenges and solution directions for enabling elastic, on-demand disaggregated memory orchestration: (1) virtual server memory and node level memory co-design and (2) local memory and remote memory co-design. A brief description of two ongoing research projects is provided for both solution directions. The paper ends with a brief discussion of other advanced and emerging memory and storage technologies and potential opportunities for memory disaggregation.