What makes a sandwich? The Earl of Sandwich, for whom the food is named, first made a sandwich with two pieces of bread and filling to keep his hands clean while playing cards. This is the standard definition; it's the essence and Platonic form of the food.
Yet there seems to be debate surrounding the topic. A post from the blog Panini Happy questions whether such items as a hamburger, hot dog, quesadilla or crepe could be considered sandwiches. I would argue that this question could be clarified with a more nuanced consideration of the definition.
Sandwich has both etymological and cultural roots in England where John Montagu, the 4th Earl of Sandwich, first ordered his meat served between two slices of bread. Here we must note that this was English bread, not flat-bread or tortilla, and the arrangement of fillings is stacked between their layers, which fits the definition of the verb to sandwich. Yet a much earlier incarnation of the dish from Medieval times used only one piece of bread: the open-faced sandwich. It is by this distinction that the sandwich transcends forms, and maybe here is the source of our present confusion.
So what is and is not a sandwich? It should follow that a hamburger is, in essence, a sandwich; it is simply meat and vegetables between two slices of bread. But what about the hot dog? Hot dog buns are long rolls sliced partially down the middle, and they don't look much like a sandwich. But in comparing this bun shape to other split-open sandwich buns like submarines and hoagies, it's clear they also fit the definition.
Quesadillas and other Mexican tortilla recipes fit the form of filling between bread, but tortillas themselves are a far cry from the soft, fat, squishy English bread and must therefore be excluded. Wraps are likewise excluded for the same reason. Crepes are made from folding toppings into thin pancakes of wheat flour, the farthest from our definition yet. (You wouldn't call an egg roll a sandwich, would you?)
There are other arguable foods: piroshki, felafel, gyro, etc, but these food items shouldn't be so disserviced by lumping them into the category of sandwich. Out of respect for their separate origins, I say let them have their own Platonic forms.