The failed missile was most likely a liquid fuel "biprop" such as LOX/kerosene or perhaps using hypergolic fuel.  Those are non-trivially difficult to design, build and fly.  As I've been saying, NK is years away from a successful liquid-fuel long range ICBM.  

Their solid-fuel efforts have been more successful, not surprisingly.  It's also not easy to build a solid fuel missile that's a true ICBM, though.  The likely scenario would be a Trident or Polaris-style relatively short-range (few hundred miles) solid fuel missile launched from a very low noise electric powered submarine.

I'm a little surprised that this failed launch has received significant publicity because it gives the lie to our government/military's litany of "FEAR NK!"