Someone who is lacking distinctive characteristics. A person that could neither be called ugly nor attractive. This could also apply to a person with a fairly neutral personality.
Most people do not have a strong distaste for cheese pizzas but would much prefer a pizza with more toppings. Cheese Pizzas usually go unnoticed because there is usually something else that is more interesting to look at. They are commonly referred to using the terms "lingerer" or "That Guy/Girl". These are the types of people whose Characters are mentioned in a film's closing credits as "Store Clerk" or "Dog Walker".
Maggie Gyllenhaal, who played Rachel in the film "The Dark Knight" is a close to perfect example of a cheese pizza (except the fact that she was cast in a leading role). The true test was when her character died at the end of the movie and no strong emotion was felt by the audience. In fact if it wasn't for the explosion that killed her, many people would not have noticed her disappearance.