Taking place in an open-world city (so, think Grand Theft Auto), players in Watch Dog will have the power to control an entire city's infrastructure via powerful computers that run everything from data records to power lines.
The player can see informational pop-ups in the world around him not that different from those glimpsed in Syndicate earlier in the year. Every character's criminal record, medical history, the works.
In sequences shown during Ubisoft's E3 conference, we could see the player blow out all mobile phone reception in his immediate area, and even short out traffic lights causing a major accident (which provided impromptu cover for an impending firefight).
There's even, believe it or not, seamless co-op that looks more interesting than just "twice as many guys shooting stuff up".
You can see gameplay footage up top, and the trailer to the left. This looks amazing.
UPDATE - Here's a teaser site for the game. There's not much there, but hey, it's there if you need it.
UPDATE 2 - While not initially disclosed, Ubisoft has since announced that the game is due on PC, PS3 and Xbox 360.