The former residents of the Queen Mansion.
When presumed dead billionaire playboy Oliver Queen, heir to the Queen fortune, returns after five years away, something felt familiar about the family's mansion. While Arrow certainly shares some cool stylistic details with Christopher Nolan's Dark Knight trilogy, Hatley Castle in British Columbia was never Wayne Manor. No, the real life location once served as the cinematic home for Charles Xavier's School for Gifted Youngsters in X-Men. And it doubled as the Luthor estate in Smallville. And it was last used in the upcoming sequel Deadpool 2.
Liam Neeson offered to appear on Arrow.
When producers and fans started talking about the forthcoming appearance of Ra's Al Ghul on the show, Liam Neeson told an interviewer he'd be interested in reprising the character, which he first played in Batman Begins. Presumably, nobody took Neeson up on his interest in order to maintain distance from Nolan's trilogy, itself separate from the DCEU, which is separate from the Arrowverse. (Neeson did turn up on TV reprising another iconc role, however, making a handful of appearances in Star Wars: The Clone Wars series.) As Arrow fans know, Ra's Al Ghul became one of the most significant and best mentors, frenemy, and big bad for Arrow. He's someone with important ties to characters like White Canary and Malcolm Merlyn, the first season's Dark Archer who briefly held the mantle of Ra's Al Ghul. The Nolan trilogy renamed his group The League of Shadows. The Arrowverse adopted the name more familiar with comic book readers: The League of Assassins.
Diggle's real life inspiration.
The Dark Knight trilogy owes much to Frank Miller's work in the comic books and in similar fashion, the TV Arrow owes a great debt to the fantastic Green Arrow: Year One. The influential mini-series was written by Andy Diggle, a British comic book writer responsible for kickass stories starring Judge Dredd, Hellblazer, The Losers, Swamp Thing, Batman, Daredevil, and The Punisher. John Diggle was named in tribute to the real-life Diggle. Eventually on Arrow, fans were introduced to Dig's wayward brother, once thought murdered by Deadshot: Andrew "Andy" Diggle.
From Green Arrow to Arrow to Green Arrow to Arrow...
Nick Fury didn't always look like Samuel L. Jackson in the comic books, but he does now. Like many great comic book based properties before it, the superb writing and casting on Arrow has in turn influenced the modern comic book iterations. Oliver's best friend, bodyguard, Team Arrow teammate Spartan, and sometime Green Arrow himself, John Diggle, was created for the TV show. He became so popular that when DC rebooted their comic continuity with the New 52, Dig was now in their pages, too.
Harley Quinn exists in the ArrowVerse.
The Joker's favorite little monster originated on Batman: The Animated Series and like Diggle, became so popular she eventually joined the comic books. Diggle was actually just a few feet away from Harley Quinn at least once, in the Season 2 episode that introduced the Arrowverse's "Suicide Squad." Despite her pointing out she's a trained therapist, she's denied the chance to come on the mission. Quinn was played by actress Cassidy Alexa who can be seen from behind in a deleted scene.
It's full of geographical Easter eggs.
Ever since Oliver's mother said she was headed to Central City and she'd be "back in a Flash," Arrow and its sister shows have been full of references to other places in the DC Universe. Now there's a multiverse of possibilities, in addition to the places we've already seen or heard about, which include a bunch of places with City at the end of the name. The Arrowverse has firmly established Coast City, home to Green Lantern in the comics. There's Midway City, home to Hawkman, Hawkgirl, and Doom Patrol. And we've even seen Hub City, the comic book home to The Question.
It's also full of real-life geography.
The Starling City skyline (or Star City, if you prefer), is made-up of footage of a number of real-life cityscapes, including Philadelphia, Boston, and Tokyo. The exterior shots of the police station are the main branch of the L.A. Public Library. The Queen Consolidated Towers are the Belgacom Towers in Brussels, Belgium.
Team Arrow loves two different bosses.
Oliver Queen is the boss of Team Arrow, but the real-life team behind the series are also in love with another boss: The Boss, Bruce Springsteen. Arrow's writers and producers have named the penultimate episode of every season after a different Springsteen song. The first five seasons were as follows: "Darkness on the Edge of Town," "Streets of Fire," "This is Your Sword," "Lost in the Flood," and "Missing."
Damien Darkh has been Green Arrow and The Flash.
Actor Neal McDonough, so great as a big bad on FX's awesome series Justified, has more than one DC related credit on his resume. In addition to playing Damien Darkh in the Arrowverse, he voiced the Flash in the Injustice: Gods Among Us video game and the Green Arrow himself in the animated short, DC Showcase: Green Arrow.
A note about 'shipping'...
A group of angry fans changed an Arrow subreddit to The Punisher as a sign of their disgust about Oliver and Felicity's Season 6 nuptials. Do you love Olicity? Hate Olicity? Do you hate all "shipping" conversations? Whatever your opinion, the soap opera ties to the Arrowverse have never been secret. Not only are there plenty of both loved and hated dense romantic entanglements throughout comic book history, but Arrowverse guru Greg Berlanti worked on Dawson's Creek and Everwood. Stephen Amell and Grant Gustin both appeared on the rebooted 90210. (The future Green Arrow lamented on Facebook at the time that he was not playing the love child of Dylan and Kelly.) It's all just part of the picture, true believers, just like the show's nerd cred. Take for example, the number of Arrowverse actors who've also been part of Doctor Who: Arthur Darvill, Colin Salmon, John Barrowman, Ben Browder, David Harewood, and Alex Kingston. The executives behind Arrow have professed their love of Doctor Who since the days of Season One.