The Flash and Arrow television series both share a number of similarities, some glaringly obvious. The worst and most unnecessary are the shows’ love triangles. Barry/Iris/Eddie reflects the majority of the key attributes of Oliver/Laurel/Tommy.
In the first season of both series, the love triangles vastly change at the last episode of the season: the dude the protagonists’ love interest is dating (Tommy for Arrow and Eddie for Flash) dies dramatically saving their sweetie (Laurel and Iris, respectively). Both Eddie and Tommy know about the hero’s abilities and are upset about keeping the secret from Laurel and Iris. I don’t like Tommy or Eddie, but you can’t disagree with them there. I don’t see how lying is protecting Laurel and Iris. They both end up finding out later, anyway.
Oliver and Laurel have these feelings for each other that they can’t indulge, due to their tragic pasts. Iris and Barry also have an overcomplicated relationship—they’re basically brother and sister. For both relationships, pride and lies get in the way of creating couples (I’m not complaining about that part: both relationships feel like they’d crash and burn).
Laurel and Iris are also shielded from the truth about Oliver and Barry’s secret identities, in part by their overprotective dads. Laurel and Iris both later find out about the truth themselves, but get over their initial anger quickly.
Oliver and Barry have short relationships on the side, partially to get over Laurel and Iris. Oliver actually has two: first with Helena Bertinelli and then with McKenna Hall. Barry has a very short-lived and infuriating relationship with Linda Park. None of these relationships last very long and they all have a fake feel to them.
Aside from their not-so-romantic relationships, Flash and Arrow have similar villains and ways of taking these villains down. Both of them face a new, minor villain (usually senseless and crazy) every week with their Big Bad remaining, for the most part, in the shadows (Slade Wilson and Reverse Flash/Doctor Wells/ Eobard Thawne—he’s got a lot of names). It’s not uncommon for their villains to try to commit suicide once they’ve lost.
In addition, both heroes have friends in the police force. Flash has Eddie and Joe, and Arrow has Detective Lance and Detective Hall.
They also have their teams of non-supers that work for and with them. Arrow’s team consists of tech genius Felicity Smoak, ex-soldier John Diggle and later he adds the unbearably annoying Sarah Lance/Black Canary to the team. She doesn’t come in until the second season, though.
Flash originally has Doctor Wells (who gives him guidance and assists him in learning about the physics and confines of Flash’s abilities) on his team, before he learns that he’s an evil, crazy and manipulative murderer from the future. He also has Caitlin Snow and Cisco Ramon on his team.
One of my main complaints about the two shows are how flat and romance-based the secondary characters are. Don’t get me wrong, the shows were super fun to watch, and Flash had one of the best portrayals of his super speed I’ve seen. But these shows were far from perfect. They really only delve into secondary characters when something involves romance, divorce, cheating and occasionally death. This becomes old fast. The only times they go into Diggle’s character in Arrow is when he’s talking about his brother’s death or getting in a new relationship.
The two TV series have complicated relationships, along with similar team structure, and somewhat cookie-cutter villains. Watching the first seasons of the series at around the same time, you realize how similar the basic concepts of the series are.