Ranking in sites like Slashdot, Reddit, and HN is a difficult and fascinating problem. I wasn't aware of the difference between Reddit's ranking methods, nor of Munroe's involvement. Thanks for the input!
If you mean difficult in the sense that it probably took some time to find the right algorithm, than you're probably right. But the algorithm itself is rather simple. I have looked at the reddit ranking algorithm and duplicated it for a project of mine. It took some iterations but the result is rather simple.