The revelation that the producers consider Nightwing the holy grail of DC characters that they'd want for an appearance on the show. Whilst it's unlikely that it'll happen due to the Batman/Superman movie and Gotham, much like the The Dark Knight Trilogy keeping Bruce Wayne out of Smallville, that the producers want the character to appear was a source of rejoicing for fans.
Felicity, Slade, and Roy becoming series regulars in Season 2. Even the promotional posters with Felicity and Diggle standing front and centre beside Oliver were enough to get people excited about the season's apparent focus on Team Arrow.
Better yet Three Ghosts ends with Barry getting his powers after being struck by lightning from the particle accelerator and falling into a bunch of chemicals.
Even better, after the series was confirmed to go to series, Stephen Amellannounced he'll be appearing in the pilot.
The decision to change the Flash's Origin Story from a Backdoor Pilot (which would have been the 20th episode of Season 2) into a more conventional pilot was praised, as it allowed Arrow to keep the focus on its core cast at that late point in the game; further, episode 18 introduced Flash-regulars Danielle Panabaker (as Caitlin Snow) and Carlos Valdes (as Cisco Ramon), which had the double-benefit of world-building as well as the Early-Bird Cameo and Mythology Gag elements.
The announcement that Summer Glau would be cast in season 2. Many are happy about this, although some are cautiously optimistic, given the 'Summer Glau Curse' and all (Doesn't hurt that the show did get another season (and more), although the SGC might not fully apply since Glau was only a recurring character this season).
Oliver taking up a Thou Shalt Not Kill policy pleased a lot of fans, especially those who didn't like his portrayal in the initial Huntress episode.
The implication in season 2's first episode that Ollie will start (eventually) calling himself Green Arrow has a lot of people pleased. After several teases in Series/The Flash through future newspapers, Oliver officially donned the Code Name starting Season 4.
Roy joining Team Arrow and starting on the path to becoming Arsenal or Red Arrow (eventually revealed to be the latter).
Amanda Waller got squees when she was announced, but when she approaches Ben Turner, offering to put him in a squad that she's forming, nerdgasms resulted at the prospect of the Suicide Squad appearing on the show.
Also4 in season 3: Freaking Ra's al-Ghul is making an appearance; as the Big Bad!
Also5in season 3:We meet Felicity's mom. This news was met with a nice reception, including the implication it will dovetail with a dive into Felicity's currently-undertold backstory.
"Guilty" saw the fan-favorite boxing glove arrow in action.
Thea finally being let in on the secret identity, given fans were getting tired of them being Locked Out of the Loop. Double points for not being a typical 'how could you lie to me this whole time!?' response, but instead a heartwarming moment between the two. The fact it then leads to the two getting ready to start training together to team up.
By that same token, Laurel finally telling Quentin the truth about Sara's death.
The news that a spinoff is in the works and Caity Lotz is being considered for the cast. Of course, given that Sara's been dead for a while now, this is more cautious optimism, but still... it's something.
Tatsu finally puts on her full Katana gear from the comics - facemask and all in This Is Your Sword.
Barry showing up in full Flash gear in the teaser for My Name is Oliver Queen.
Ray is working on making the A.T.O.M suit shrink.
Stephen Amell debuting his new costume as Green Arrow live at San Diego Comic-Con. Watch it here.
John Constantine is not only making an appearance, but will also play a key part in Sara Lance's resurrection, pleasing the Constantine and Arrow fandom a TON. Bonus points for bringing Matt Ryan back.
Curtis Holt/ Mr. Terrific will be a gay african american man, while his comic book counterpart is straight.
As of the season five mid-season finale, three words: Laurel. Is. Back.note Then promptly undone by the preview for the next episode revealing it's really Earth-2 Laurel pulling a Dead Person Impersonation
It takes until "Fighting Fire with Fire" near the end of Season 5, but Curtis finally gets his T-spheres, officially taking a level in badass and becoming the Mr. Terrific fans have been waiting for. Especially considering that he's been criticized since his introduction for being an Adaptational Wimp.
Katie Cassidy/Black Siren being announced to return for the finale of Season 5 of Arrow, and then the news that she was upped to Series Regular status of Season 6!
Oh dear Chase has captured all of Oliver's team and is holding his son as a hostage. He'll need to recruit some unlikely people to help like Malcolm Merlyn and Nyssa Al Ghul man that's awesome....wait is that SLADE WILSON?! Cue big fandom squee!
Colton Haynes returning as a series regular for Season 7. Other than one Fridge Horrornote His character Roy is last seen leaving Star City with Thea and Nyssa, heavily suggesting something bad happened to the latter two hence why he returns surrounding his return, fans universally rejoiced when it is announced.
The announcement that Marc Guggenheim and Wendy Mericle will no longer be show runners after Season 6! Considering how much the show's quality has gone down under the duo, it's hugely satisfying!