According to the GaWOC based in Loughborough Univ. in the UK, which ranks cities' importance, ATL is a gamma/minor world city. The GaWOC is generally considered to be the authority on such matters.
The levels on the GaWOC and a few represenatives of each level:
Alpha/World "Capital" City (NYC, Tokyo, London, Paris, LA, Chicago, etc.)
Beta/Major World City (SanFran, Moscow, Hong Kong, etc.)
Gamma/Minor World City (ATL, Boston, Washinton DC, Miami, etc.)
Some evidence of becoming a world city (Richmond, Charlotte, etc.)
Things that would move ATL higher:
-Better transportation system
-An event held in a city often and/or landmark known throughout the world (like Indy 500 in Indianapolis or the Statue of Liberty in NYC)
-More business headquarters
-a livelier nightlife
-a livelier downtown
-a denser, more definite urban core
-better smog management
-more parks and other public facilities
-more "traveling" events to be held here (like a Super Bowl or a concert)
-an international event held here (like the Olympics or a G-8 type of conference)
-better sprawl mangement
-good infrastructure
-a distinct "identity" (like NYC is "The City that never sleeps")
Things not list, but I'd like to see, and could probably help its standing:
-A denser skyline
-Better relations w/ the South
-The BeltLine