In addition to just the networking hardware, the networking stacks and topologies are very different. I would consider something a supercomputer if it was using a high-dimension torus topology, and some MPI variant, and a different internet / transport protocol (I forget the name, but as I understand it's a very different approach where there's much less decision making in the routers). On the other hand, what I would consider to be a cluster is almost going to use off-the-shelf networking hardware (as you mention), TCP/IP, and a simpler hierarchical topology with a switch or two per rack.