Recommended Video: Prioritizing Your Product Backlog
YouTube: Value vs. Complexity & The Product Management Trap
While pretty basic, this video serves as a good reminder when reviewing your backlog and prioritizing your work. If you wanted to get more sophisticated you might consider adding profit numbers (through new sales or customer retention) as a third axis since there are plenty of times when a customer may think something is valuable but may not be willing to pay for it. At the very least, this would help sub prioritize the features/requests within the top quadrant. Knowing a specific dollar amount may not be worth the effort but a scale of 1-5 may help.
There are a number of factors that one can use as axis variables to create this matrix besides value and difficulty. As described in this article about the pitfalls of using ROI to prioritize your backlog, you need to find the metrics that make the best sense for your organization and can be somewhat stable across releases.