Seems like a bit of a loaded question. It’s pretty hard to pin down my least favorite thing about Mormonism as a religion. I love it! I believe it!
That said, there are cultural aspects associated with Mormonism that annoy me, particularly the Utah brand of Mormon subculture. They tend to be somewhat exclusive and can leave non-Mormons feeling left out. Things like premarital sex and pregnancy or smoking can often lead to ostracism.
Or just not being a mormon. My cousin isn’t mormon and it was almost impossible for him to get a job in the little town he lived in.