In the U.S., one cannot falsely scream "Fire!" in a crowded theater or threaten the President or slander someone, but otherwise freedom of speech extends quite far — to the extent that individuals (and corporations) are entitled to say and write things that cause deep resentment, fear, and anger among other people. See Brandenburg v. Ohio. This freedom is included in the Universal Declaration of Human Rights of the United Nations, but it's watered down in the corresponding International Covenant on Civil and Political Rights which notes "The exercise of the [right] carries with it special duties and responsibilities. It may therefore be subject to certain restrictions, but these shall only be such as are provided by law and are necessary for respect of the rights or reputations of others [or] the protection of national security or of public order or of public health or morals." Many western European nations have adopted such restrictions (e.g. Sweden, Germany, and the United Kingdom.)
Despite the conditions of the ICCPR, nineteen members of the UN have not signed up. Among them are the predominantly Muslim countries of Brunei, Malaysia, Oman, Qatar, Saudia Arabia, and the United Arab Emirates.
I've never read an issue of Charlie Hebdo. From what I can tell, it oscillates between the mere irreverence of the National Lampoon and the deliberate defamation of religions. Accordingly, columnist Rabbi Marc Gellman declines to join in je suis Charlie. On the other hand, British Prime Minister David Cameron leans toward the U.S. position— as does the Mayor of Rotterdam, a Muslim.
I have read several media reports of polls and surveys to the effect that roughly half of Muslims in the U.S. and more than half living in western Europe consider Islam to be incompatible with western values. It's noteworthy that an even higher percentage of non-Muslim westerners say the same thing. These polls and surveys ask generic questions with poorly defined terminology, and their methods are suspect. I wish a pollster would make the question quite specific: within the context of free speech — even the ICCPR version — that facilitates open expression of Islam as a minority religion in many western countries, are 99.9999% of Muslims willing to forego violence as retaliation for what they perceive to be blasphemy? The Rev. Dr. George M. Clifford explains that such violence is not mandated by the Koran itself. And are 99.9999% of non-Muslims in the west — including profit-seeking journalists and attention-seeking entertainers — willing to show restraint and respect by not trashing the beliefs of one-quarter of the world?
If the answers to both questions are No, the coming decades will be ugly.