Back to the original question: Mormons are quasi-Christians, so why should other Christians be more critical of them than non-Christians?
Well, in some ways, this is not surprising. In religion, politics, and many other walks of life, it’s the SMALL differences that cause major schisms and hostility, not the big ones. It’s as if we expect so little of outsiders, as if we hold them in such disdain that they’re not even worthy of our hatred.
Think about it- is there any Democrat (except, maybe Ted Kennedy) that conservative Republicans hate as much as they hate John McCain?
WHo gets angriest at Reform Jews? Not Christians! No, it’s the Orthodox Jews who hate them most!
SO, it’s not surprising that Jews and atheists show no special animosity to Moromons. To atheists, ALL religions are equally stupid, which makes Mormons no worse than any of the other idiots! And to Jews, ALL Christians are deluded dolts worshipping a dead carpenter- so they bear no special animosity against the Mormons.
It stands to reason that only people who take Christianity very seriously are going to object to the teachings of other Christian Churches! Hence, fundamentalists are going to hate the Catholic CHurch (which they see as corrupting the teachings of Christ) in a way that no Hindu ever could (indeed, the Hindu might well think the differences among Christian sects are negligible). Similarly, since Christians tend to ignore Moslems, they can be more “tolerant” of the differences between Shiites and Sunnis… whereas the Moslems, who take the differences seriously, are much more likely to fight over them (even though, to an outsider, it would SEEM the two sides have much in common).
If a Catholic (or Baptist, or any other Christian) takes his faith and his church seriously, he’s BOUND to be uncomfortable with the Mormons’ claim that Christianity is incomplete, and needs fulfillment by Joseph SMith’s Book of Mormon. In the same way, Jews probably resent the idea that their religion (which they think is pretty wonderful as it is) needed “fulfillment” by Jesus.