Terrorists and criminals free to roam and make mischief.
Schengen has a central database in Strasbourg. The police of every member country will have access to it. If you're a criminal or terrorist thinking the new Europe means you get around scot-free, the same information sharing means that the police in every country know who you are. There's freedom of movement around the EU but the trade-off is the disappearance of the cloak of national anonymity.
"Show me just what Mohammed brought that was new, and there you will find things only evil and inhuman, such as his command to spread by the sword the faith he preached." - Manuel II Palelologus