I have several Mormon friends. My biggest problem with it is that, effectively, you buy your way into heaven. You can't get into heaven unless you have your secret name, you can't get your secret name unless you receive some particular sacrament, you can't get that sacrament unless you're a good Mormon, and you can't be a good Mormon unless you tithe. (And tithe actually means a full 10% to them.)

There are other problems, too, like baptising non-Christians in absentia and post mortem, even after having been asked to stop. That's just rude.

Random wackiness, like secret underwear, I couldn't care less about. It's mostly the strongarming people into paying money that's my issue with it. And that's my same issue with Scientology. Admittedly, Scientology is a thousand times worse about it, but it's the same scam, ultimately.