I'm the OP... let me first say how great it is to see all this activity in the thread. I file my Market Report to Overstreet in December every year (and it gets published the following July), but I'm always keeping an eye on the discussion here.
The original rationale was that the existing lists in Overstreet (as has been referenced above) were based solely on price, which meant that for things like Bronze and Copper books, it was always rare variants (like the 2D edition of Miracleman 3D) that dominated those lists. To my mind, this doesn't tell me a great deal about the overall market, and my mandate from Overstreet was to try to increase late bronze, copper and modern content in the market reports.
I try to cover lots of ideas and trends (and I'm always open to feedback), but my sense was that an ever-evolving list of what the marketplace saw as "keys" would be of interest, as it mixes ideas of market demand, content appreciation, and the cultural "zeitgeist" in a way that a numerical price list doesn't. And rather than just using my own list, I though it would be cool to leverage the collective knowledge of these boards to support that effort.
The result is a list that I'm sure nobody is 100% in agreement with (including me), but that hopefully represents a general consensus about what the Copper Age keys are... or at least the beginnings of a consensus.
The fascinating thing for me is that the list does change from year to year. For a time, Apocalypse appearances really were "hot" - and they climbed onto the list. That's clearly not the case now, but as the hobby changes, so does the list.
It is (at best) an imperfect exercise, but I feel that input and discussion can only improve it, and the OSPG more broadly.