Water chemistry is another one. I've lost count of the homebrewers I have known who couldn't believe I used barely-treated tap water (when several excellent local breweries have done the same thing) and then were stunned by the quality of my beer made without complex recipe-specific water profiles.
BTW, I used to fret about my water until I moved and lost all my salts and just had to "wing it" for a couple brews. I have seen little evidence that a complex water profile improves anything over "get the right pH range and be done with it."
I can believe you experience, based on what water you have. Most around here say you don't have to do anything, but they are on the water system that uses Water from Lake Huron. My town uses well water that tastes chalky, and is only good for stouts and porters.
So I am that guy who buys RO water, adjusts and checks with a pH meter. Many thought I was a geek, until my beers improved. I have coached many in the club on how to make their pilsners really shine by getting rid of the alkalinity that gave a muddy finish.
So yeah, I have an experience where people said you don't have to do that, then it became "How did you do that?".