The Punisher made his first appearance in the pages of The Amazing Spider-Man # 129 in February 1974. He was originally going to be a minor second string character due to his penchant for killing, but due to his popularity, he became a more prominent figure in the larger Marvel Universe.
In all shots of the New York skyline the Stark building is missing as it should be near the Empire State Building as it was featured in both avengers movies and Spider-Man Homecoming. This is due to the fact that they are considered to be in different "universes".
When Billy Russo is calling for Frank on the radio, Frank's call sign is Raven. In the comics, Frank is a Vietnam vet who is the best at killing and warfare, much like John Rambo, who, when his former colonel is trying to call him on the radio in the movie First Blood is revealed that his call sign is Raven.
While waiting for Frank in the woods, David (Micro) Lieberman is seen reading the book Life of Pi by Yann Martel. Earlier, when Frank visits Lieberman's house, Leo (Micro's daughter) discusses the book with Frank while repairing the car. She then gives him her copy to read.
The only 2 Marvel Cinematic Universe characters to sustain the most wounds during fights and kills are Punisher and Daredevil. While they both lack super powers, their combat skills make them the fiercest. Frank was trained in the Marines while Matt Murdock received his training from his fellow blind mentor, Stick.
The trivia items below may give away important plot points.
This is the first time that Jigsaw/Billy Russo does not appear as he is depicted in the comics: a gangster and mobster. However, he appears as the founding CEO of ANVIL, a private military company. In addition, he appears as Frank's best friend whilst in the comics and in Punisher: War Zone (2008), he is Castle's arch nemesis.
Frank Castle and Billy Russo are shown reading "Moby Dick" and "The Picture of Dorian Gray", respectively. These choices hint at their personalities: much like Captain Ahab, Frank is driven by an obsessive quest for revenge, while Billy puts too much value in his good looks (foreshadowing his final confrontation with Castle, which leaves him permanently disfigured). Ben Barnes, who plays Billy Russo, played Dorian Gray in the 2009 movie, Dorian Gray.
The only Marvel/Netflix series not to feature Rosario Dawson's Claire Temple character so far. Rosario Dawson said on an interview that the producers tried to arrange a cameo but it wasn't possible due to schedule problems.
On episode four, after Castle kills everyone in the warehouse to get the red mustang, you can see a man lying dead on a pinball machine. Diamond Select Toys sells a figurine of the Punisher with a man laying on a Spider-Man pinball machine, which they state they modeled off of art work from Tim Bradstreet.
Claire Temple (Rosario Dawson) has appeared in five of the six Netflix Marvel shows. She didn't make an appearance in The Punisher while Turk Barrett (Rob Morgan) has appeared in all six. He makes an appearance in Iron Fist season 2.
During the scene where Billy Russo first delivers the rent money to Curtis, Curtis refers to Billy as "a man of wealth and taste." This is the second line from "Sympathy for the Devil" by the Rolling Stones. In the song, this is said by the devil, foreshadowing that Billy will be the antagonist.