There is nothing wrong with multiculturalism. In fact, it's wonderful. It makes the world a better place. It enriches everybody. But wholesale, indiscriminate, across-the-board, reckless multiculturalism is incomplete. It is missing one simple distinction, and that makes it blind.
Multiculturalism is great. Blind multiculturalism can get us into trouble.
The missing distinction is mutuality. It's great to tolerate other religions or ethnic customs if the people following those religions and customs also tolerate ours. It is self-inflicted abuse to tolerate them when they do not tolerate ours. And it is cultural suicide to tolerate a religion that actively tries to undermine or destroy ours.
It is a crime against humanist values to let them be taken away (one small concession at a time) by a less-tolerant culture simply because the less-tolerant culture is more insistent, aggressive, and relentless.
Read more: Multiculturalism and the Defense of Liberty.