DC Universe Online is a Massively Multiplayer Online Role-playing Game (MMORPG) set in the DC Universe. SOE hopes to make a different kind of MMORPG, with The Incredible Hulk: Ultimate Destruction being cited as one of the main inspirations for gameplay. SOE is working to make DC Universe Online more interactive than standard MMORPGs, while trying to keep key elements of RPGs, which include a leveling system and inventories. The world is mainly shared, public space instead of heroes or villains owning territory. The public space features dynamically-generated content designed for both hero and villain player characters, which may lead to interesting Player Versus Player battles[[I]citation needed].
It is known that players choose a city for their character; the currently known cities being Metropolis and Gotham. The character is given a Justice League or Legion of Doom Communicator depending on their choice. For example, a hero character may encounter a thug robbing an armored truck while a villain character will instead rob the armor truck. It is also known that player characters have threat ratings which will cause heroes and villains to interact with the character[[I]citation needed].
IGN previewed a demo video, showing Brainiac infecting citizens with a virus that causes them to riot in the streets. Lex Luthor decides to test out his battle armor and Bizzaro comes to "help". Superman, along with Supergirl and Green Lantern, helps the 'player' defeat the infected citizens and remove the nanovirus. It's not yet known whether this mission will appear in the final game.

Links:
http://www.gametrailers.com/player/36413.html (Trailer)
http://uk.ps3.ign.com/objects/755/755358.html (Game info)
http://en.wikipedia.org/wiki/DC_Universe_(video_game (General info)