Mainly network, as others have said. But there are also scientific applications that are more appropriate for distributed computing (don't need the fast interco

Mainly network, as others have said. But there are also scientific applications that are more appropriate for distributed computing (don't need the fast interconnect), but get run on supercomputers anyway because of data locality (post-processing/analysis) or grant structures. The cool thing here (hopefully) is that it makes it easy to have both: HPC/supercomputer for jobs that need that and cheaper/easier cloud resources for jobs that don't.