The American sandwich lives in a world unto itself. It's certainly not like anything you'd expect in Europe.
The first obvious difference in the experience is the sheer number of questions that you'll face when ordering what you thought was a simple food item. What kind of bread, what kind of spread, eat here or takeaway, toasted? It becomes an inquisition, and you often wonder what exactly it was that you did wrong.
The concept of butter on a sandwich is alien. It just doesn't exist. I don't like mayonnaise, so I usually take my sandwich dry.
The size of the sandwich is another consideration. Not only is the containing bread usually oversize, but the amount of filling seems to be the result of a competitive spirit in the deli-worker. I counted fourteen slices of ham in my sandwich the other day. I've tried asking people to hold back, but they insist on loading the thing up.
I'm also almost always asked whether I want a pickle with my sandwich.
I almost always answer "no".