whats intelligent about a game thats just a more violent GTA without the cars?
Manhunt is less violent than GTA. Like you said, there are no cars or other vehicles, so you can't run over pedestrials on purpose. You don't get to mug up or kill innocent civilians, like grannies. You can't go postal on a killing spree on a shopping center or snipe at people from a sky scraper. You don't get to steal, sell drugs, abuse prostitutes or in any way run a criminal empire. But that's not really the point.
The point is, unlike for example Quake or Counter Strike, Manhunt does not glorify or trivialize killing of people. On the contrary, it underlines America's macabre adoration of violence - the multi billion business of offering consumers violence as entertainment. In Manhunt you don't get the feeling that you are a good person because you kill or mug another human being (e.g. a supposed terrorist, granny or some generic evil being). You don't get to place the disgusting pleasure you get from killing people under some self-deceptive hypocritical banner of "doing the right thing".
Manhunt is satire.
It's also well done satire - characters are done with great artistic taste and some of them are very funny. For example, to this day I have not heard better voice acting in any other game.
What comes to game play, it works. But that's not what the controversy is about.
-AF