It's mostly trolls, but there are legitimate concerns about Islam.
While Christianity has become tame since the Enlightenment, Islamic countries have not. It's only Muslims who live in countries in the west which have become moderate, but even then many are justifiably concerned about Muslim communities due to the amount of extremism and fundamentalism which evidently does exist. Whichever way you spin it, the majority of Muslims support Sharia and other traditions which the West has fought against over the last century or two. I suppose that's why some people don't like it.