When peskywhistpaw and I began to put the list together, what we generally looked for in a non-rare pairing was overall presence. (Admittedly, we were really iffy on a number of them, so what we ended up doing was just sticking some of them on the list anyway in the hopes that others would help us to refine the list.)

What I mean by "overall presence" is the number of existing fics for the pairing, whether it be on LJ or FF.net or wherever. Since a numerical limit would be difficult to keep track of, I guess the task at hand is to come up with a way to categorize all the middle-of-the-road ships as rare and not rare.