Part of the reason for this is undoubtedly his reputation for unapologetic sentimentality (almost completely absent in crime fiction since the "tough guy" 1950s) and his propensity for O. Henry–style twist endings — both aspects that appear dated to readers in the 21st century.
But alongside those traits runs an ironic (often pitch black) sense of humor that would probably surprise someone expecting a more "innocent" tone from 1930s fiction. Take for example this (more than somewhat edited) selection from the story "Sense of Humor" (available along with many others in the new collection Guys and Dolls and Other Writings):
"Say," [Joe the Joker] says, "I am going to play a wonderful joke on Frankie Ferocious."Add to that Runyon's signature use of present tense (which can come off as stilted when he forces past- and future-tense phrases into its rules but has a charm all its own), and you have something that is difficult to get into at first, but that is immensely rewarding once you "learn the language."
"Well, Joe," I say, "you are not asking me for advice, but I am going to give you some free gratis, and for nothing. Do not play any jokes on Frankie Ferocious, as I hear he [...] will not laugh if you have Al Jolson, Eddie Cantor, Ed Wynn, and Joe Cook telling him jokes all at once. In fact," I say, "I hear he is a tough audience."
"I am going to have myself delivered to Frankie Ferocious in a sack."
"Well," I say, "personally, I see no percentage in being delivered to Frankie Ferocious in a sack, because as near as I can make out from what I read in the papers, there is no future for a guy in a sack that goes to Frankie Ferocious. What I cannot figure out," I say, "is where the joke on Frankie comes in."
"Why," Joe the Joker says, "the joke is I will not be asleep in the sack, and my hands will not be tied, and in each of my hands I will have a John Roscoe, so when the sack is delivered to Frankie Ferocious and I pop out blasting away, can you not imagine his astonishment?"
Well, I can imagine this, all right. In fact, when I get to thinking of the look of surprise that is bound to come to Frankie Ferocious's face when Joe the Joker comes out of the sack I have to laugh, and Joe the Joker laughs right along with me.
"Sense of Humor" ends with a twist worthy of the darkest crime fiction and had me open-mouthed, but at the same time, it's really the only ending that fits the story. Runyon's stories often wrap around themselves and tie up neatly with a ribbon, often in a way that elicits a smile at the author's craftsmanship.