Hey- what about this- considering that the "Harvey Dent Act" really does sound like a specific piece of legislation, and that photo from the New York set that refers to Gotham citizens seeing "something" and saying "something, what if the Harvey Dent Act is a specific law, that makes it illegal to NOT report a crime? The Harvey Dent Act could be a law that makes people essentially accomplices for any crime they know about and don't report.
Here's why I think that might be it- Harvey Dent was all about confronting crime and not being afraid of the mob in open court. The idea that people could be forced to report crime- maybe be held criminally liable for the crimes they fault to report- would certainly crack down on crime. But it would cause a huge amount of mistrust between many people- and that could be the "seething" people are talking about. So now, you have a populace that is safer, with less crime, but many, many people are frustrated by feeling like the have been forced to betray people close to them for committing crimes. Friendships and relationships ended, people distrust each other a lot more.
A law like this would go against our society's laws and many in the "real world" would consider a duty to report crime or face punishment a breach of their liberties. But in the fantasy world of Gotham city, with all of the crazy stuff that has been going on, it might be something that is enacted as an emergency measure the people accept. I can imagine the feeling of rage that people would feel after having to live under a law like this, report on people close to them for committing any crimes- even petty ones- and THEN finding out that the person who's ideology it was based on was in fact breaking the law himself by killing people - including cops- with the flip of a coin...
Thoughts anyone?