Great article from LA Weekly.
And I ask you... what is the Quesadilla but a melted cheese taco. Am I right? You know I am.
The only requirement is that it have cheese (aka queso). These days anything goes and some are so filled with goodies, they might as well be burritos.
The best darn Quesadilla I have had to date is the Vampiro at Mexicali Taco in Downtown, LA. I tend to avoid melted cheese. It's bad for the butt bags. But it's seriously delicious and great for kids and great for vegetarians.
When I was catering, two favorite appetizers I created back in the 90's were the Blueberry Brie Quesadilla and the Goat Cheese Pear & Caramelized Onion Quesadilla. Good Stuff.