I read once that Jamie Oliver cooks these for the family dinner more than any other dish. I can see why because they're simple, quick, can be made with any number of ingredients and, most importantly of all, they're very tasty. Crisp tortilla holding oozing cheese and anything else you fancy.
The two ingredients you can't do without in a quesadilla are cheese (queso) and tortilla - which I'm guessing is where they got there name. Outside of that you can put in whatever really, Wahaca even puts mashed spud in one of theirs along with some chorizo and thyme. Being a meat-aholic my most cooked filling is chicken but beans, chorizo or just cheese goes down well - as long as you perk it up with some onion, coriander and chili that is.