Islam.
I won't participate in a farce of a thread that can't even summon the guts to mention its main worry, but so soon on the heels of Lee Rigby being beheaded with knives on a London street, there's a certain elephant in the room. Nothing can be done with such ineffectual fainted-hearted mush that goes 45 posts with nothing on-point to the real concerns in everyone's minds. I'll just say that...
This may come as a shock to you, but Islam is not something that registers on my list of concerns. I have good friends who are Muslim, I've visited a Mosque, and I've done readings from the Qurʼan. Nothing in it scares me. Nothing about the worshipers I've met and come to know has scared me. Even if we're talking about extremists, I may not agree with them, but I can understand the reasons they exist, and how they can bring themselves to do what they do.
Religious Extremism is a fear-based reaction, driven by a misguided form of Self Preservation; they are told by other members of their faith to fear that their religion and way of life is being threatened, and that desperate methods are required to "save it" from others. The violence and desperation we see in extremists are a very human reaction to an insidious lie, one that's easy for those in the throes of fear to believe.
Such a belief is difficult to combat, but warfare, deportation, equally extreme actions taken by "Anti-Muslim" hate groups, and so on will only fuel the fear that the world is against them, and that even more desperate methods are required to save themselves and their faith. You have to show these people that there's nothing to fear... that you do not want their way of life destroyed, do not wish ill upon them, and that you really do respect and value their teachings, even if you may disagree with them. For those on the fence regarding an extremist cause, it will show them that there is a place for them in the world, and that there is no need for such desperation and panic. For the most violent, militant extremists among them, it will reveal them to be the misguided fools and wrongdoers to their followers, and to the rest of the world.
Islam is not a problem, but a religion. Extremism is not a problem, but a symptom of fear, isolation, and persecution. It sounds corny as hell, but the way to cure this symptom is through tolerance, efforts to understand them, and through open communication that reaches across geographic and language boundaries (such as through online communities like this one).