I'd call scale-out an interesting inflection point where products with older "design centers" will be overtaken overall by products that are built with scale out in their "design center".
I think that it starts with the software architecture - the algorithms - and that the architecture imposes constraints on the hardware. As Rickover said, "The devil is in the details, but so is salvation."
@fdmts Here you go then. Scale out = An innovative, game changing, and leading solution that leverages actionable synergies across unique, optimized paradigm shifts to deliver bottom line results that you can double click on.
At the compute and storage layer, it's mostly software. You can build some massive scale out systems using standard white boxes. For networking, there are benefits to using custom ASICS to minimize latencies.
@dinisco would usually see this with Infiniband before 10 GigE based architectures. Of course Infiniband could usually drive lower latency more predictably.