^ No, you raised a valid point. Most Protestants, Catholics and Orthodox at least recognize each others' "Christianity," even if they think their own version is the most "correct." (There are some exceptions, like the fundies who think the Catholic church is Satanic, but they're pretty much on the fringes.)
Mormons are quite different, because they alone among the major sects have added a whole new set of scriptures which they place on an equal footing (or actually a superior footing) to the Bible. I would still consider them Christian, since that's how they see themselves, but their new scriptures and elevation of Joseph Smith to the status of prophet - and source of divine revelation - does set them apart in the eyes of most other Christians.