Few ingredients, excellent ingredients, and the restraint to leave them alone.
Italian cooking is defined by restraint. A great Italian dish typically has four or five ingredients and no technique that draws attention to itself. The skill is in sourcing and in knowing when to stop.
The flavor base is olive oil, garlic, tomato, basil, and cheese in the south; butter, cream, rice, and cured pork in the north. Acidity comes from tomato, lemon, and wine; umami from parmesan, anchovy, and cured meat.
Adding too many ingredients to a dish built on restraint.
Serving sauce on top of pasta instead of tossing it through.
Using pre-grated parmesan.
Breaking spaghetti to fit the pan.
Rinsing pasta after draining.
Adding cheese to a hot pan and making it stringy.