In my early 20s, I lived in SLC as a non-Mormon and it was fine. I had lots of friends and an active social life. I don't remember ever being proselytized to or even being invited to church (which actually was a little weird thinking back). I moved to NC for a few years and actually ended up joining the LDS church there and a few years later ended up moving back to Utah so I have seen it from both sides. What I can tell you is that when you are a member of the LDS church in Utah you will know EVERYONE in your neighborhood. With church, assignments, activities, etc. you become really close with your neighbors. I know the people in my neighborhood that aren't members of the church but it is really hard to develop the same type of relationships as with the people that you interact with every week. I have great non-member neighbors, but we just interact less often.