To slang, or not to slang?
First, it’s important to make a distinction between slang and genre dialect. If you’re writing crime noir set during the Depression, your readers are going to expect some gaudy patter about gats and dames and gin joints. And if your novel is set in the Old West, there’s going to be a lot of shootin’ and ridin’ and “pardner” this and “stranger” that.
But you have to strike a balance — an excess of slang within dialect will easily tip homage into parody. And, prithee, don’t clutter your medieval romance or high fantasy with feverishly filigreed language that would require subtitles if it were to be adapted to film.
A more immediate danger, however, is in incorporating contemporary lingo or catchphrases into fiction that takes place in the present or in nonfiction about a current topic. Ten years on, “Not that there’s anything wrong with that” or “Not so much” still work, but they’re getting stale. Soon, they’ll likely be as irrelevant as those everything-but-the-kitchen-sink parody movies like Scary Movie and Epic Movie, where look-alike cameos of pop-culture 15-minute wonders pop up randomly, with hilarity presumably ensuing.
Some years back, I read a detective novel featuring a black private eye in which the narrative and the dialogue alike were laced with “Aaaiiiiight” and other African American elocution. It was less annoying than you might think (the authenticity was actually refreshing), but the novelty was just that — in retrospect, an ephemeral affectation.
The medium for which you write will determine the shelf life of slang. Online publication can be almost instantaneous, but it lives on forever, and the slang therein soon loses its luster. Newspaper readers are more forgiving, because they’re consuming the paper’s news and feature articles in the moment, but a magazine article sees print up to several months after submission (and people are likely to hold on to periodicals than papers), and books and film scripts are written a year or more before they hit the shelves and screens.
Some slang lives forever — booze and “beat it” are each hundreds of years old — and occasionally a slang term (mob, for example, or lousy) — becomes part of standard speech, but be prudent about incorporating slang into your writing; individual writers who nominate rad or groovy for posterity when something with the staying power of cool is available risk distracting their readers with an unintentional verbal equivalent of a speed bump.
Slang can also be misinterpreted, or may at least interfere with comprehension, as language shifts. The context will probably help your readers understand what you mean by tranny, but as the diminutive of transvestite overtakes that for transmission in terms of frequency of use, any employment of it for the latter meaning outside of a car magazine may elicit an interruption of concentration.
The ubiquity of slang in spoken discourse inures us to it, but when it is sealed in print or online like a pressed flower, it may lose its bloom, so chill, aaaiiiiight?