As Wikipedia notes,
St. Louis-style barbecue sauce is generally tomato-based, thinned slightly with vinegar, sweet and spicy. It is not as sweet and thick as Kansas City-style barbecue sauce, nor as spicy-hot and thin as Texas-style. A St. Louis-style barbecue is not complete without copious amounts of sauce.
That is what makes St. Louis-style sauce so good. Just the right combination of sweet and spicy in a sauce just thick enough to properly coat the meat.