Well, what is it?
Is it the bread?
The toppings, e.g. lettuce, tomato, sprouts, etc.?
The spread, e.g. mayo, mustard, [insert exotic name] dressing?
Well, of course it's probably a combination of all (and probably some additional) factors listed above.
But, the question I pose is what's the most important thing to making a sandwich great?
For example, with burgers it's pretty easy to say that the patty is the key ingredient. We can all lard our burgers with cheeses, ketchup, mustard, bacon, etc., but it all comes down to the hamburger patty. If the patty ain't good, the best bacon in the world is only going to get you a BLT, not a hamburger.
So, is there a similar defining ingredient for a sandwich?