Mass Effect 2 does have New Game+
Or you can just go back in after the end.
BioWare has incorporated "New Game+" into Mass Effect 2, despite originally ditching it because it would make it harder to handle the transition to Mass Effect 3.
New Game+ allows players who complete a game to replay it using the same character with which they finished it, retaining abilities and so on.
The process is complicated here because the Mass Effect games are intended as a continuous narrative, with episode two (and eventually three) able to inherit decisions made by players in the original adventure via a save-game import feature.
"Yes, originally we removed New Game Plus from Mass Effect 2 early in the development process," community man Chris Priestly explained on the BioWare forum. "However, we decided later that it was a valuable addition to the game then looked at ways of implementing it back into the game.
"After thorough design and testing we felt that the feature was ready to be placed back into Mass Effect 2 and implemented it again."
New Game+ isn't the only option once you finish Mass Effect 2 though. In order to better support downloadable content and to "tie up any loose ends", e.g. side missions, BioWare will allow you to continue playing after the end of the game without having to go back to the beginning of the main story.
Mass Effect 2 is out for PC and Xbox 360 on 29th January and our review will be available around then.