I've said it before but I think it bears repeating: The foundation of Mexican food is corn, chiles and beans. If you don't understand these three ingredients, you won't get very far. I think you can judge a Mexican restaurant by the quality of these three items and the technique in making them a finished dish.
I have friends in New York who complain about this or that Mexican restaurant because the flavored margaritas aren't very good. Or the guacamole wasn't made tableside. The real secret is an understanding of chiles, beans and corn, if you ask me.