Almost all has way too much salt. Unfortunately, you often get as much or nearly as much as the maximum daily recommended amount in just one fast food meal.
The drinks (sodas) are the next problem. High fructose corn syrup is used instead of sugar, and that is even worse than sugar. Or if you get "diet" drinks, the sweeteners are cancer causing AND they fool your body satiation mechanism, so you end up EATING MORE.
Those are the main things, Also, "super-sizing" and fat content.