Giorgio Milos is master barista for the high-end Italian coffee company illycafe of Trieste and maintains a blog at the Atlantic. In his first post there last May, he criticized American baristi for not knowing their business; in particular, he took them to task for not knowing how much coffee to use when making espresso.
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!