This is a great question. To say that after 911 our world changed is an understatement. In light of this, law enforcement has to go through some important changes. Let me name a few of the challenges.
First, police work has to be based on more information and this information now has to be shared among the various agencies. This will require more intellectual sharing infrastructure among the various agencies.
Second, due to the threat of terrorism, the role of the police has increased in important ways. For example, who would have thought that planes could be flown into buildings? Other examples include the infamous underwear bomb and the shoe bomb. As crimes get more creative, the police needs to be aware of more things.
Third, another great challenge for the police is to keep in tact the civil liberties of the people, while at the same time doing their job. This can be very controversial.