Atlas Park is considered the heart and center of Paragon City in the now-defunct City of Heroes MMO. It is highlighted by its signature statue of Atlas (actual scale, according to historians) holding a globe on his shoulders, which can be seen in front of City Hall.
This is considered the starting point for all registered heroes in Paragon City, as all transit busses drop off the heroes at Atlas Plaza in front of the statue. New heroes will meet their first contacts and engage in their first missions here. It is also the only place where heroes would have access to Fort Trident, headquarters of the Freedom Phalanx.
City Hall is the home of city government as well as the home of the Federal Bureau for Super-powered Affairs. Initially the duties of the FBSA were divided between City Hall and the Freedom Corps headquarters in Galaxy City, but with the Devastation of Galaxy City in 2011, those responsibilities were consolidated in City Hall.
City Hall is where a hero would create a supergroup and pay for monthly upkeep. It is also where you would see the City Representative, who would traditionally authorize you to both wear a cape and utilize auras. It is also the place where you would have access to the special portal that would take you to Recluse's Victory.
(For more information of the zone including detailed in-game information, visit the Paragon Wiki entry.)