The biggest mistake I've seen is an enormous quantity of coffee being used - way too much. I'm talking about 20 to 25 grams of coffee for a single espresso shot! It is like making a mojito with half a mint leaf, one ice cube, a few grains of sugar, and a gallon of rum. Undrinkable!
In a post today, Milos says that another year of touring high-end U.S. coffee shops has convinced him that we're turning things around here, but he reminds would-be coffee mavens that the goal is to "create an ideal experience in the cup rather than 'I did it because I could.'" He concludes by asking them to "resist the urge to over-concentrate the coffee - to 'go bold.'"
I said it last year, and I'll say it again: the right beans in the right proportions do a beautiful job all on their own. It applies to French press, pour-over (individual drip), vacuum siphon, and any other method, just as it applies to espresso.