The most immediate problem with fast food as I see it is that it encourages messiness in all aspects of consumption. A burger drips butter down your wrist, while the cucumber is trying to escape from the trapdoor below. The toppings on pizzas know all about gravitational acceleration. Atleast bhel puri plates have the humble piece-of-cardboard to help scoop up the mixture, but the pani in the puri loves spurting out, especially if you're a novice at it.
As we know from experience, no one bothers a bit about hygiene, nor do I pay too much attention to it. But if only someone would invent a less messy way to eat these things.