These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: It's Always Sunny in Philadelphia
Alternative Character Interpretation: Occurs in-universe and lampshaded/discussed in "The Nightman Cometh"; the rest of the gang notes multiple times that Charlie's play reads more like the account of a child being molested by an adult in his sleep than the romantic epic fantasy he envisioned. This also raises some distressing questions about Charlie and his relationship with his increasingly creepy uncle both in and out of universe.
Badass Decay: Inverted with Frank, who becomes more insane and Crazy Awesome with each episode. Subverted with Mac, who, despite his delusions, was never badassed to begin with.
Big Lipped Alligator Moment: The giant sea turtle floating in outer space during the end credits of Charlie Rules the World. However, it's a Brick Joke to one of Frank's line that is easy to forget. Still, it's just way too weird of an ending.
Broken Base: A lot of people are upset by the fact that by season nine, Mac's basically canonly gay. Some people also think that it's an invention of the later seasons, despite the fact that there's an implication of his homosexuality right off the bat in the pilot episode.
Despite only showing up a few times so far, the Lawyer has gained a following due to being one of the only outsiders who's gotten a handle on how to deal with the gang's outrageous behavior, trumping them consistently and effortlessly since the entire gang is legally clueless.
Friendly Fandoms: With Pacific Rim, due to the presence of Charlie Day (and possibly the fact that Newt is basically a marginally-smarter version of Charlie...).
Growing the Beard: The show really came into its own during the second season, adding Danny DeVito as a contrast to the main four, turning Dee away from the Closer to Earth trope and generally going for more manic, darker, exaggerated tone.
Hollywood Homely: A Running Gag. Dee is actually pretty hot, but the rest of the Gang and her mother talk about her like she's the ugliest person on the face of the Earth. Lampshaded in "Who Got Dee Pregnant?", when the Gang's attempts to find the father of Dee's baby results in them discovering that Dee's getting more ass than all of them combined.
Idiot Plot: Most of, if not all, of the plots. It helps that they're all just the kind of idiots the plot requires.
Magnificent Bastard: The Lawyer. In one episode, the gang repeatedly pesters him for different legal reasons, like for patents and a job contract, their tactics involve assaulting his secretary, stalking him, hiring him a hooker, threatening to rape his wife (as an act of good faith), up until, he gives in, resolves all their troubles legally, and gets the inventions to patent. Then at the end, he reveals that the contracts everyone signed gave him the profits from any and all sales of the inventions, as well as a restraining order from them (and back-up copies because the first was eaten in an attempt to stop him).
Nearly every aspect of Charlie and Frank's living habits. Charlie has such bad oral hygiene that he can effortlessly pull out his teeth without feeling anything, and he presumably doesn't change or wash his underwear as it was easy for someone to pull out after a failed attempt at a wedgie. Frank then poops wherever he feels like it because it's funny to him, and he clips his toenails with a steak knife and uses the same knife to peel fruit.
They Wasted a Perfectly Good Plot: In "Who Got Dee Pregnant?" the episode begins with the guys discussing a plan to spend the night at the city museum, dodging security and generally enjoying themselves. Though the episode was also funny, it's a shame this never came to pass.
Charlie, the Butt Monkey is often so pathetic that the viewer is intended to feel sorry for him and the gang is even moved to pity him on a few occasions. In contrast, Dee is another Butt Monkey, but she's so shrill and spiteful that she never gets any woobie moments.
Rickety Cricket, who is introduced this way and grows steadily worse with each appearance.