They take all of the subtle, wonderful flavors that the ingredients have individually, and put them into one big flavor mess that tastes mostly like bread and whatever the strongest ingredient is.
Who invented this crap? I can't even taste the cheese, or meat, or tomato usually. I just taste bread, and whatever sauce is on it. What a waste of cheese and meat. Why bother?
Also, people who work at sub shops and don't care are also evil. I hate them. They can single handedly ruin your dining experience.
Example: I just went to the Lil' Dinos on campus. It's a sub shop. A cheap one. I got an 8" Spicy Italian on wheat. I asked for olives, lettuce, tomato, banana peppers, and oil and vinegar. The guy working there (Indian graduate student) gave me a total of one tomato, 3 slivers of banana pepper, enough olive pieces to MAYBE make one whole olive, and then didn't shake the oil/vinegar bottle. So basically I just got oil. And then he didn't cut it all the way. He got through the top layer of bread and just stopped. I had to tear the meat and the other piece of bread myself.
It's not my fault you're a graduate student who has to work in a sub shop; make my damn sandwich like you give a damn, my dinner is at stake.
edit: Jersey Mike's is the exception to the sandwiches are evil rule. Because Jersey Mike's is awesome and makes really good subs that actually taste like they have ingredients. Plus they know how to put toppings on.