Creator Backlash: A mild example, compared to others — Stephen Amell has quite happily gone on record with his displeasure over the marketing for Season Two...namely that the posters highlight the ensemble cast, but leave out Paul Blackthorne and Susanna Thompson.
Diggle's unborn child has already been named Diglett.
Deathstroke and Brother Blood's Mirikuru-empowered minions are referred to as the Deathstrokes, or some variant of, by many fans, though aren't actually named in the show itself. One of the alternative names is 'The Blood Cult'.
Hey, It's That Guy!: Let's start off with saying that somebody on Arrow must really have liked Spartacus, no less than three actors from that show have shown up. Manu Bennett, Katrina Law, and Cynthia Addai-Robinson all appeared on Spartacus before moving here. Especially interesting is the fact that, unlike other examples of actor-overlap,Spartacus was filmed half a world away in New Zealand.
Poor Kaitlin's family members have a nasty habit of dying.
A number of people suggested that due to her brown hair not being a trait inherited from two blonde parents, Thea wasn't Robert and Moira Queen's daughter. It was revealed in "State v. Queen" that Thea was conceived after an affair between Moira and Malcolm Merlyn.
Malcolm is Back from the Dead. Okay, technically he never died in the first place, but the point stands.
As soon as Sara asked Oliver "What happened to Slade?" without mentioning Shado, people correctly assumed she was killed.
Slade's return. Given the character's popularity, most fans knew he wouldn't be killed like he was. Also, since the character is known for being a villain, as soon as they mentioned what Ivo was looking for and what Brother Blood was doing, many worked out he would be used on Slade and that he was the Big Bad well ahead of the reveal.
One of Us: Head writers Guggenheim and Kreisberg are big comic fans and love continuity gags.
Mr. Blank was originally supposed to be the DC Comics villain Onomatopoeia. However, as Onomatopoeia only speaks in, well, onomatopoeia, they couldn't make it work without him coming across as mentally retarded (since sound effects sound far different in real life compared to comic books).
The twentieth episode of Season Two was originally going to be a Back Door Pilot for a potential spin-off focusing on The Flash. Based on both the strength of Grant Gustin's portrayal of Barry Allen in "The Scientist" and "Three Ghosts" and the fact that using the twentieth episode as a Back Door Pilot would interrupt the show's momentum as it built up to the season finale, the decision was made to make it a standalone pilot.
In early drafts of Season Two, Sara was originally going to take the role of Ravager, but eventually ended up as the Canary. Instead, the role of Ravager was given to Isabel Rochev.
The character who was going to buy out Queen Consolidated was originally going to be Ted Kord, a character whose existence was already implied through the appearance of Kord Industries. However, DC enacted an embargo on him due to "other plans", so the creators instead went with Ray Palmer.
Technically its still a case of what could be, but Liam Neeson has said he'd play the show's version of Ra's Al-Ghul "in a heartbeat."