In the past day, I've run across two related theories of how all of Quentin Tarantino's films are part of the same universe: this video and this post on Reddit. They differ slightly but the Reddit one is more interesting...specifically that Pulp Fiction, Reservoir Dogs, etc. take place in the aftermath of Inglourious Basterds and its unorthodox ending to World War II.
Because World War 2 ended in a movie theater, everybody lends greater significance to pop culture, hence why seemingly everybody has Abed-level knowledge of movies and TV. Likewise, because America won World War 2 in one concentrated act of hyperviolent slaughter, Americans as a whole are more desensitized to that sort of thing. Hence why Butch is unfazed by killing two people, Mr. White and Mr. Pink take a pragmatic approach to killing in their line of work, Esmerelda the cab driver is obsessed with death, etc.
You can extrapolate this further when you realize that Tarantino's movies are technically two universes - he's gone on record as saying that Kill Bill and From Dusk 'Til Dawn take place in a 'movie movie universe'; that is, they're movies that characters from the Pulp Fiction, Reservoir Dogs, True Romance, and Death Proof universe would go to see in theaters. (Kill Bill, after all, is basically Fox Force Five, right on down to Mia Wallace playing the title role.)