What are the secrets to good sub sandwiches?

All the delis in NYC use basically the same ingredients: Boar's Head meat and cheese, iceberg lettuce, red onions, etc. But some of them make sandwiches that are just fine and some of them make sandwiches that are mind blowing. What's the difference? Is it all in the bread? Is there a secret to slicing the veggies? Have they perfected the ratios? Does it matter what order things are layered in?