I have a perception that Western European Christianity (in fact European Christianity in general) was unusually intolerant of other religions, and I wanted to know if I was correct or whether this is overblown. While Islamic societies had a degree of toleration for their non-Muslim subjects (specifically Christians and Hindus), Christian societies seemed totally intolerant of other religions, with subjects typically facing far more pressure to convert than those in other societies in similar circumstances, e.g. the Christianisation of Northern and Eastern Europe, and of the Americas. A notable exception might be Norman Sicily.
Was European Christianity's religious intolerance and missionary zeal unusual? What motivated it?
I think that answer to your question is both yes and no. A lot of the perception that the Middle ages and Christianity were more brutal and less tolerant stems partially from enlightenment historiography. Christians did do a lot of bad thinks to people of other faiths, such as attacks on the Rhineland Jews at the start of the first crusade and the killing of almost everyone in Jerusalem, Christians, Jews, and Muslims alike at its end.
Treatment of the Jews by Christians during this time has a lot to do with the history of early Christianity, where early Christians not only wanted to distinguish themselves from the Jews, but also blamed them for Christ's death. In addition, while Islam has a command calling for the tolerance of Jews and Christians, Christianity does not have such a command.
However, Christians lived peacefully along side many people of other faiths just as often. In fact, in Christian Spain many of the laws forbidding Jews from leaving their homes on certain christian holidays or the laws against Jews and Gentiles eating together or marrying were either already forbidden by Jewish law or put into effect to protect Jews from Christians. In addition, the local church was often responsible for overseeing and protecting the Jews against lay Christians. Also, going back to the first crusade, while he was calling for the second St. Bernard of Clairvaux explicitly included in his letters that the Jews were to be left alone to try and avoid another massacre.
I also think a lot of people like compare Medieval Islam against Medieval Christianity in the question of tolerance. Muslim polities were generally very tolerant of religious minorities for many reasons, one of them being that Muhammad called for the protection of the dhimmi, or Christians and Jews. However, there is additional incentive for a Muslim ruler to tolerate the dhimmi in that the dhimmi are to pay the jizya, a special tax on Jews and Christians. There were also plenty of Muslims who did terrible things to people of other faiths, such as Almohavids in southern Spain from the 12th to the 13th century. The Almohavids killed none Muslims, forced conversions, and generally treated none Muslims far worse than previous Muslim kingdoms had.
So to answer you question, by modern standards, Medieval Christianity is certainly less tolerant, but it was not really less tolerant than Islamic societies, who could be just as harsh.
Sources: The Crusades: A Reader and the Siete Partidas (a law code from 13th century Castile which contains laws concerning Jews.)