null vs. undefined and their behaviour in JavaScript

So after a big argument/debate/discussion on the implementation of null and undefined in javascript I'd like somebody to explain the reasoning behind the implementation and why they differ in some circumstances. Some particular points I find troubling:

• null == undefined evaluates to true
• null + 1 equals 1 but undefined + 1 equal NaN
• if(!null) evaluates to true and if(null) evaluates to false but null == false evaluates to false.

I've read the specification and I know how the results are reached, I'm looking for the paradigms and reasons that dictate this being the specification. Some of these points, especially the second one, given the first, feel very inconsistent.