Yeah, I'm most familiar with Utah/Idaho Mormons who tend to be on the more extreme spectrum. The problem is that these are the guys who hold the power in the Church.
Something that hasn't been mentioned is that Mormons give 10% of their income to the church. AFAIK, the vast majority goes into building and maintaining churches and large, extravagant temples, and a very small portion actually goes to real charitable work. If you could convince a Mormon to leave the church but still donate 10% of his income to a more effective charity, I think you'd end up doing a lot more good from a utilitarian perspective.
It's also pretty well-known that the state of Utah has an abnormally high rate of mental illness, and a lot of people suggest that Mormons often claim that they're a lot happier than they really are. I tend to agree: as a Mormon, you're taught all the time about how the Church is so great and how it's the best path to happiness, so you're naturally going to want to appear happy to other people.
My hunch is that Mormonism tends to make certain people a lot happier and other people more depressed. I definitely fell into the latter group--when I was Mormon I was constantly wracked with guilt because I--gasp--masturbated. At the same time, because I held the Mormon priesthood, I was essentially told that I had magical powers, and that I had more authority to act in God's name than the Pope. So I think for some people Mormonism is a big guilt trip, and for others it's a big power trip. Both are unhealthy.
I am an ex-Mormon who attended BYU. Your description of Mormons is completely foreign to me.
This is completely true, although it's probably worth pointing out that Mormons believe marriage is eternal, so in the afterlife polygamy still exists.
I grew up Mormon and attended BYU for a few years, and a lot of descriptions of Mormons I read on here are completely foreign to me. Knowing that the LDS Church was literally true was always an extremely important aspect of the religion when I grew up--it wasn't just about the community.
I suspect that the types of Mormons that people on LessWrong tend to come in contact with are very much outside the mainstream. While I can see that Mormon theology can be twisted to support a sort of trans-humanism, in my experience the typical Utah Mormon would find this very bizzarre.
To be fair to Mormons, they don't have the concept of an eternal hell like most Christian fundamentalists. There is something called "outer darkness", but you'd have to work really, really hard to get there--like, harder than Hitler.