Great article. One nit though. When making a 2d Cartesian graph with the two axis there are accepted quadrant labeling practices [1]. Quadrant I is positive on both axis, Quadrants II and IV positive on just one axis, and Quadrant III is negative on both. What you described as Quadrant IV should really have been Quadrant III. You can flip axis any which way, but the opposite quadrants will always have 1 step in between them. No big deal, but will make little sense if somebody actually tries to visualize your model.
Thanks. On the numbering of quadrants, I'm aware of that. The numbering is an order based on the value to the business.
First Quadrant (interesting/essential) is never an issue to allocate. In the ideal world, all work would be 1st quadrant. It's important and good for morale.
Second Quadrant (undesirable/essential) is what companies have to do. It's a morale liability, but too important not to get it done.
Third Quadrant (interesting/discretionary) is what one hopes will be achieved with slack in the schedule. Ideally, it'll pay off in the long term. It's good for morale, but not relevant to the immediate path-of-execution.
Fourth Quadrant (undesirable/discretionary) is the junk that should never exist.
Originally I used "category" rather than "quadrant", but quadrant implies a certain visualization, even if the numbering is different.
[1] http://en.wikipedia.org/wiki/Quadrant_(plane_geometry)