Overcooked vegetables: Steaming veggies until they’re mush kills the flavor and removes many of the nutrients.
Overcooked meat: It’s already dead. There’s no need to cook it so long that all the flavor is depleted. If one prefers a charred surface (which is potentially hazardous due to carcinogens), then quickly broil/flame the meat, leaving the inside tender and juicy. (Of course, no one should serve or eat meat which is dangerously underdone.)
Gargantuan portions: Few things turn me off to eating in a restaurant as when I face a huge plate overloaded with food. Not only does it make consuming the meal seem more like a chore than a pleasure, but it dampens the prospect of dessert.
Miniscule portions: The ultimate in culinary pretense, and often parodied, is the expensive restaurant that serves a tiny wedge of food, surrounded by empty plate surface. Skimpiness is not desireable.
Misuse of spices: The purpose of herbs, spices, and the like is to supplement the natural flavor of food. It’s not to drown out the flavor and bring attention to itself. Nor is it to overwhelm people’s senses and cause them physical distress (not only when the food is entering the body, but as it exits as well). A good indicator of whether food is too spiced is whether you want to take another bite, or you become desperate for a glass of ice water. Any restaurant that boats of serving chicken wings so hot that a customer is required to sign a release form before consuming them should be shuttered by the health department.