It's more about tenancy from what I understand. You can give unlimited people access to their own private, encrypted virtual 10.0.0.0/8 block all in the same data center and SDN makes that possible by encapsulating packets in another layer.
One area where a lot of smarts are needed needed, for example, is making sure that you're intelligently routing encrypted packets to the right machines without reverting to a broadcast to every router, switch, and machine in the DC.
I've also heard a lot about people running brand new untested protocols (like TCP replacements) right alongside traditional protocols through SDN.
I'm probably going to do a much worse job of explaining things than the guys who presented at the Open Networking Summit [1]. I highly recommend giving the keynotes a listen.
[1] https://www.youtube.com/user/OpenNetSummit
Edit: I do think this relates to SDN more than a traditional router/switch OS, based on the comments about "traditional and programmatic, declarative control plane." Either way, though, the link above is definitely good stuff.