#3 N. Korea has a not so friendly China to it's north and the world's most well defended border with a 600,000+ S. Korean military setting behind it to it's south.  Iraq was pretty much open except for the small part of the border with Kuwait.#4 Part of going into Iraq was to cerate change in the middle east.  Taking out N. Korea wouldn't change anything in the far east. China isn't hard core communist any more and S. Korea and Japan have been stable democracies far a long time.