Rocketry is grotesquely difficult, which is why they respectfully call it rocket science.
So the North Korean achievement is impressive, considering their vast poverty. Unlike anything they have flown before, the Taepo Dong-2 apparently has four motors in the first stage. That makes it much more difficult to build. Preventing vibration from one of the motors from interfering with another motor is not easy. And since it has a longer range than anything they have used before, it is a big challenge to get it remotely close to the target.