These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Amanda Waller staring down Batman is probably the big one (loosely adapted into the Justice League Unlimited cartoon with Waller offhandedly calling him "rich boy"), but the gradual reveal of Oracle proved to be the one with the most disproportionate effect on the DCU.
For perspective, following her crippling, Barbara Gordon had been all but written out of the Bat-books and the DCU in general, and most people remembered her as Yvonne Craig in the Batman TV show from the 60s. After this, Oracle's become big enough to get her own TV show, several comic series, repeat Justice League memberships, and inspired the resurrection of a cheesy villain named the Calculator just to be her evil counterpart. She's generally considered one of the sexiest women in the DCU, and it's primarily for her personality and mind.
Deadshot. The series put the character on the map (he had less than a dozen pre-Squad appearances) and is largely credited for his popularity amongst fans. Because of this, he became the team's de facto field leader in the New 52 series.
The squad itself in appearances in other books. Virtually every time they've shown up in guest spots, it's someone's depiction of a "classic" line-up, leading to people hoping for a revival.
Nightmare Fuel: As of issue 6, Yo-yo is still alive inside King Shark's stomach and he appears briefly to beg for help.
Rescued from the Scrappy Heap: The guy's name is Captain Boomerang, for crying out loud, yet he's one of a handful of characters to survive the whole series while going on most every mission, even ones where his teammates actively try to kill him.
Tainted by the Preview: The New 52 reboot of Suicide Squad didn't make the best first impression. A lot of the people who would have eagerly supported such a book were put off by Secret Six getting cancelled seemingly to be replaced by the new Squad, and the new costumes (particularly for Harley Quinn) were... not well-received. Since the book officially launched, it has attracted a fair share of complaints for its characterizations that seem a bit off and for, what some consider to be, pointless design changes, such as Amanda Waller's slimming-up.
Broken Base: The Joker being in the movie, primarily because of not being part of the Squad in the comics. However, a notable amount of fans are excited for Jared Leto and his interpretation of the role, or are just interested in how it'll play out. Late 2014 reports suggest he'll be a supporting antagonist, which if true would make complaints about him being a member of the Squad moot (although the validity of these is yet to be confirmed).
Like Heath Ledger before him, Jared Leto has his work cut out for him to establish his own version of the Joker without being seen as too derivative of his immediate predecessor. At the least, Leto has a Academy Award to his name before he got the part (for Best Supporting Actor, the same category where Ledger won posthumously for his Joker performance), so his ability to act isn't so much the question as much as the impact he can leave as the character.
To a lesser extent, Will Smith as Deadshot and Jai Courtney as Captain Boomerang following after Michael Rowe and Nick Tarabay, respectively, in Arrow.
Margot Robbie was fortunate enough to avert this, due to portraying a character that hasn't appeared in the film medium previously (although Harley did feature in the short-lived Birds of Prey tv series) and receiving acclaim in her recent projects. Before leaving, Tom Hardy as Rick Flag was able to avert this for the same reasons, and because he'd already proved himself to DC fans as Bane in The Dark Knight Rises.
WTH, Casting Agency?: While most of the people were good with the main cast when they were announced, a few were surprised that Will Smith landed the role of Deadshot, most likely due to the Race Lift involved; since initial buzz for the film centered on the actors approached while the characters were yet unknown, some expected he was going to play Black Mantanote Though he was only revealed to be black several years after his debut. or another black supervillain. Meanwhile, some are just unsure if he can pull off a villainous role that's so heavily against type.
Fans who were concerned that Warner Bros. would play it safe by churning out movies starring Superman, Batman, and the Justice League were pleasantly surprised to hear that this was the first project that they would release on after Batman v Superman: Dawn of Justice hits theaters. Many fans are happy that they're trying to do something different with their shared universe.
The announced cast has also been generally well-received in contrast to the casting of Batman, Wonder Woman, and Lex Luthor, which all proved immediately contentious among fans for various reasons.