Following-up on a couple of observations from above:

1. While O&D traffic may not be as great in ATL relative to LAX, NYC, or even Chicago, the potential connecting traffic in ATL is greater than any of those. If you look at domestic air travel patterns in the U.S., the majority of it can fit into two categories: a) East-West traffic (i.e., basically traffic from the eastern U.S. to points in the western U.S) and b) Northern U.S.-to-Florida. ATL and ORD both have value as connecting points, whereas LAX and the New York airports have very little connecting point value other than for international flights. Of ATL and ORD, ATL is ideal for connecting all sorts of points in the Northeast and Midwest to Florida, while ORD is not. For example, you would really be going out of the way to connect via ORD flying PVD-MIA. So, the better geographic ability to connect results in a slight nod to ATL and probably accounts for the final rankings.

2. The 5th runway is under construction now at ATL. While originally designed as a short 6000 foot runway to handle ASA prop planes and get them out of the jet approaches, the runway now will be built as a full length, 9000 foot runway. The post above suggests that this final length is a future project, but the fact is that the project was redesigned well before construction began and now will be built to the full length. My understanding is that it will allow three aircraft to land simultaneously. The longer runway was required in large part because it is not that easy to land an RJ in 6000 feet (it can be done, but at a cost of excessive braking), so the longer runway is basically a way to save wear and tear on the RJ fleet.