Why does Oliver have a Domino Mask painted on? So it doesn't fall off in the heat of combat.
A modern alternative is shades, but that leads to Sunglasses at Night, was used on Smallville, and doesn't allow Ollie to emote simply by pulling his hood back.
Why doesn't Oliver have his iconic goatee from the comics? It'd be a big giveaway that he's the Arrow if they had the same distinctive facial hair.
To follow up on this, Oliver even shaves off his Perma Stubble after rescuing Walter, because Walter saw that much of his face, and might have ended up making a connection.
Ollie's Inner Monologue stops after the first few episodes. In-Universe, this is because Ollie is adjusting to no longer being alone on the island and having only himself for company. Also, he was nearly caught by Diggle speaking his thoughts aloud. Narratively, it's because Ollie now has Diggle and others to confide in and the plot is developed, so the monologue is no longer required to convey his inner thoughts.
Malcolm Merlyn's plan seems to revolve around tearing down the social institutions of Starling City, like closing the free clinic his late wife started. When did he cut off Tommy? Right after Tommy ran a fundraiser for the CNRI, a free legal aid clinic.
Malcolm's archery skills and fighting ability originally sem to be a bit of an ass pull, but then you remember, he's a very rich man who is shown earlier in the series to be into fencing, a sport often practiced by rich socialites, as is archery.
There's also the fact that he trained for two years in Nanda Parbat, but we don't find that out until after he kicks Ollie's butt about.
Dark Archer and Hood aren't just archtype counterparts of each other. Their lives and histories are that as well. Malcolm lost his wife, an unequivocally good person, and became a dark vigilante. Ollie lost his father, an unequivocally evil person, and become a good one. Their family members who aren't in on the secret are in many ways, angry and upset about all the secrets. And so on and so forth.
All of the stories relating to Oliver's time on the island in season one could be pretty much interpreted as Green Arrow: Year One. Though Year One stories from DC tend to be about the first year of a hero's career (and therefore imply that season one is essentially Year One), in the context of the show the island stories end up as Oliver's Year One tales. Year One's often juxtapose our hero from what we know them as to what they were before they began, and the island is considered as Oliver's true beginning.
Oliver's not a great liar but the stories he tells Felicity before recruiting her are particularly bad. Why? He's testing her to make sure she is not going to go to the police and to see if she'd be willing to help out.
It's been noted that as well as other possible influences, the finale of season one in which Malcolm Merlyn plans to level The Glades killing off everyone unfortunate enough to live there, seems to be somewhat familiar to the end plot of Batman Begins. Given that season two strongly implies the source from where Malcolm received his training is Ra's Al Ghul himself (who was also revealed to be the main villain of Batman Begins) wiping out a significant portion of people for the "good" of others is exactly the kind of mindset that Ra's Al Ghul would approve of and enforce in any students.
"State v. Queen" reveals Moira's case is unwinnable when Laurel discovers Adam's trump card against the defence. After revealing it to Moira, Moira comes clean to her children about her one-time affair with Malcolm Merlyn years prior, after which she admits in the trial and becomes the main focus of the prosecution's cross-examination. However, the information about the affair doesn't appear as earth-shattering a reveal we're led to believe, maybe because it was Moira who chose to reveal it prior to the testimony... That is, until the end, when Malcolm reveals that Moira had been hiding the fact that Thea was his daughter, and therefore Oliver's half-sister. This was the damaging information Adam had uncovered on Moira and would have used had he remained on the case. Out of the family's best interest Laurel focused her cross-examination on the affair alone, knowing that this fact would hurt Oliver and Thea.
This also provides a new depth to her actions after the cross-examination, particularly her reaction when Oliver tries to check if she's okay.
Why was Laurel made assistant and then lead counsel for the prosecution in Moira's case despite being a clear violation of legal ethics and protocol? Malcolm Merlyn arranged it as a backup plan to ensure Moira's acquittal. It would have created reversible legal error on appeal if he hadn't been able to sway the jury.
Barry Allen mentions seeing a blur with a man inside it on the night of his mother's death. This can refer to both Professor Zoom or Flashpoint. On the Fridge Horror side, that potentially means Barry witnessed his mother's death.
How exactly did Ollie get that bank account-hacking arrow? And who's the idiot who wouldn't get suspicious about him wanting it? It's doubtful he made it himself, as he's never shown much competence when it comes to software.
Ollie has been shown to have connections within the Russian mafia, among other potential as-yet-unknown connections. It wouldn't be too unlikely that he could obtain it through more covert means than ordering it through Queen Industries.
Is it even a bank-hacking arrow? All we see is that it links to Hall's computer via wi-fi. Could just be basically a wifi-hub attached to the arrow, and the actual hacking was done on the main computer back at the cave. Which still doesn't explain how Ollie did the Hacking, but hey, Early-Installment Weirdness.
Perhaps Oliver already knew how to hack computers before hand, he was a bit of a tearaway afterall, but likely, the arrow just uploaded some kind of spyware onto his computer that Oliver used to get the information to access his bank account, just as many real life hackers do (just, you know, without any arrows or rooftop jumping involved). So, in short, it didn't hack the bank account and do this for him, it just stole his bank account information and Oliver used this to steal his money and give it to CNRI.
However, he is shown going to Felicity for most of his computer needs. Plus, he has been gone for a while, and technology has changed in five years.
Where did Helena get her incredible fighting skills? Everyone else has a reason except for her. Training sure, but no one in her close circle of family even suspected she might be anything other than a rich princess and that kind of training, as we've seen, is a long hard process.
Its possible Frank Bertinelli had her take extreme fighting training so as to avoid her ever being in danger and used to undermine him.
One of the tie-in digital comics is devoted to Helena learning how to fight and kill from an ancient Italian organization of assassins after her initial appearance. If referring to her return episode, then this training would be where her skill came from.
Also, one "Behind the Scenes" video mentioned that they somewhat modified her fighting style between ep 7 and 8 to imply that Ollie gave her some training beside archery training
Why did the producers think changing, Sara's hair color was a good idea? Did they not realize that the plot hole it created in Season 1 where her mother was looking for a brunette?
The Other Darrin, no more, no less. Made very jarring by Jacqueline Wood and Caity Lotz looking nothing alike, but they evidently hadn't thought that far ahead when casting Sarah for the pilot. I mean, it was a year between the two, and since their writing is pretty spontaneous, they hadn't thought of making Sarah Black Canary yet. Or they had and Wood didn't want to come back. So they cast someone who looks more like her comic counterpart. Who happened to be blonde. Since they're very obviously two different people, they just didn't bother giving Lotz dark hair.
Why did they go to the trouble of drugging and kidnapping Barry? He's going to find out Oliver's the Vigilante anyway and it's not like they're going to stop him from walking out and finding out where the cave is, so wouldn't it be less trouble to just pick him up and explain things on the car ride over?