This very short post about why software-defined networking is "cool" makes a comparison to the evolution of Ethernet protocols.
As an example of what SDN can do for users, consider one of the best and worst parts of Ethernet. Spanning Tree... While this was extremely handy 20 years ago, it now limits network engineers ability to build high performance networks because spanning tree (and LACP bonding) place limits on how many parallel paths there can be between switches. Contrast this with Infiniband, where truly massive fabrics with full bandwidth between all endpoints are trivial to construct and manage.
-- Phil Pokorny
Read the rest of Phil's article (it should only take 2 minutes) to see why this evolution is similar to the movement toward SDN.