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.
Bart is a major divider. From the time he went from Impulse to Kid Flash, to becoming the Flash, to his death. And now he's back to being Kid Flash and has his original Impulse personality, people are still complaining. Why? Because they want him back as purely Impulse and see Wally's daughter being Impulse III being "an obstacle for Bart to return to form". Already some Bart fanboys are losing it and getting very angry with Iris claiming his former codename...
New 52 Wally West with his Race Lift, completely different backstory, and everything.
Mostly between Barry and Wally fans; Jay fans are mostly spared this, as there's a general consensus as to where and what he should be. (Or at last there used to be, before New52. With Jay now out of the main continuity and completely rebooted onto another Earth, who knows?)
The New 52 has caused a Broken Base of San Andreas Fault proportions, what with Wally, Linda, their kids, Johnny, Jessie, and Max all now absent and apparently never having existed (although a very different version of Wally eventually appears), Barry's marriage gone, Bart and Jay effectively different people, and all of this being Barry's fault.
The Barry Allen Flash's Arch-Enemy is Eobard Thawne, the Reverse Flash, AKA Professor Zoom. An obsessive fan of Barry, Eobard managed to duplicate Barry's powers. He then attempted to replace Barry, before stalking, and apparently murdering his wife, Iris Allen, when she rejected his advances. Eventually he drives Barry to kill him to save his new fiancÚ before he can kill her. However, Thawne was resurrected years later, and has gone out of his way to cross every line he can find since. Using his time travel powers to edit his life, Thawne kills his brother (who he blamed for holding him back), his parents (who weren't supportive of his obsession with the Speed Force), a professional rival, and every man that a woman he is interested in has ever dated; he then traumatises her as a child when she still spurns him. He kills Johnny Quick shortly after his own resurrection, tries to force Barry Allen, newly returned to life, into becoming the Black Flash, and during the Flashpoint crossover, alters the time stream further and further to make Barry's life as miserable as possible. In addition, in The Flash: Rebirth he kills Allen's mother, among other acts, thus indirectly causing Flashpoint. He has no Freudian Excuse or mitigating factor for these actions, but instead he's just a selfish man who plans to get what he wants at the expense of everyone else.
The most infamous criminal in the history of Gorilla City, Grodd was driven into exile after killing the city's benevolent leaders and ruthlessly plundered the mind of his friend Solovar to discover the secrets of mind control. Combining a ruthless brilliance with a gorilla's power, Grodd established himself as a feared player of the underworld. He would brutally murder anyone who stood in his way or just dispose of humans for the enjoyment, considering them far inferior to himself. Grodd's mental prowess is such that he enjoys dominating and twisting the minds of others to feral savagery or into sycophants for his ego. In battle, he is also highly sadistic - for example, crippling Hunter Zolomon, who recalled with terror how Grodd treated him like a toy he could break. To date, Grodd has made no fewer than eighteen attempts to completely wipe out all of humanity to purge what he sees as a lesser race.
Copy Cat Sue: A canon example (sort of). Barry Allen seemingly returned from the dead in 1993, but it turned out it was Eobard Thawne before he became Professor Zoom. Eobard Thawne was such a fan of Barry Allen, he memorized Barry's biography, spent years and his fortune to aquire the same powers, and had himself surgically altered to look exactly like Barry (using futuristic plastic surgery.) But he ended up arriving years after Barry's death and, addled from time travel, and the revelation that he would become Professor Zoom, snapped and started thinking he was Barry Allen. One can only imagine how well Barry would have reacted to the arrival of an obsessive Fan Boy who looks and dresses exactly like him and has his powers. If only DC did actual "What ifs".
Wally, faced with the prospect of Professor Zoom killing and replacing Barry, throws off 10+ years of psychosomatic blocks on his top speed and decks Zoom. The look on Zoom's face alone makes it worthwhile.
Wally becoming the first person to ever enter the Speed Force and return to kick Kobra's ass with a slew of new abilities, becoming the fastest man to have ever existed.
Wally again, this time defeating the Black Flash (the avatar of death meant to claim the lives of speedsters) by outrunning it to the end of time where death no longer has any meaning. To make it even better, he brings Linda back from the Speed Force on his return trip to the present.
The Rogues destroying their evil unprincipled counterparts during Final Crisis.
Shirley Walker's music for the two-hour pilot and all the episodes (Walker became the first woman to be entrusted with a show of this nature - and it was her work on this that led to her doing Batman: The Animated Series).
Growing the Beard: Vol. 2 (Wally's series) started getting stubble with William Messner-Loebs's run, in which Wally started toning down his cockiness and his girlfriend Linda Park was introduced. Then Mark Waid came on and the beard had officially grown out.
Internet Backdraft: Who's faster? The Flash or Superman? Power level discussions involving the Flash tend to not be pretty.
Professor Zoom. He killed Iris by phasing his hand into her head, and when he a woman he was interested in who denied his advances, he tortured her till she was reduced to a vegtable and killed everyman she every dated.
Hunter Zolomon as Zoom adopts this trope. He me murders innocent people close to superheroes, will not hesitate to kill anybody who gets in his way, and he thinks that his actions are improving other heroes. He truly thought that he was helping Wally when he tried to kill his pregnant wife.
Replacement Scrappy: Every single speedster has suffered this in someway. Jay Garrick is the only one with immunity, only because he was the first.
Wally for Barry, at first.
Bart's brief tenure as the Flash, which was thankfully reversed.
Iris claimed the name of Impulse and fanboys have raged as result.
The second Trickster.
Post Rebirth, Barry for Wally. It also doesn't help that DC decided to demote Wally's role to the point where his current status in their new reboot wasn't even shown for two & a half years.
Patty Spivot for Iris West as Barry's love interest in the New 52.
The New 52 incarnations of Wally & Bart for their pre-Flashpoint incarnations.
Gorilla Grodd: After decades of attempts at turning everyone into apes, Geoff Johns had him single-handedly demolish several city blocks before going on to give Zoom his Start of Darkness.
Johns did this for the Rogues in general, whom Mark Waid had written out due to finding them cheesy. Bear in mind that their leader wears an Eskimo costume, most aren't into anything heavier than bank robbery even after the Dark Age of comics, and were considered jokes even before then. Johns' Rogue Profiles one-shots redeemed most of the characters in fans' eyes.
Sequel Displacement: Despite the fact that he was the first, Jay Garrick is nowhere near as well known in popular culture as Barry Allen.
Originally played straight but then averted with Wally West; when he was the Flash, he became the most well known at the time in part because of his appearances in Justice League, until recent years when Barry came back and DC began pushing him to make fans forget about Wally. Even now, among comic circles its still debated on who is the 'iconic' Flash.
Spotlight-Stealing Squad and/or Creator's Pet: For some people, Barry Allen under Geoff Johns, whose favorite character is Barry. You'll often hear one specific panel referred to in discussions of this issue, in which someone comments to Barry that the crime rate has gone way up since he's been dead... despite the presence of Wally, a far faster and more powerful Flash (facts even Johns acknowledges).
Take That, Scrappy!: While Barry isn't really hated (except as a Replacement Scrappy) as he is, the fact that he caused Flashpoint, which caused the New 52 gets him a lot of crap from people who don't like it. So, of course, Future Flash, a future Barry Allen, says that if it wasn't for him, Wally would be married and have two kids, and that it's his fault Wally is dead. While the latter is more of guilt for not saving Wally, it's uncanny how this dialogue matches up with complaints about Barry's actions.
Wally being changed to a black guy has caused quite a bit of controversy. Mainly because many think he will either become an Ethnic Scrappy, or will stay the same, in which case they feel the change was unnecessary.
With the annual now out, and Wally is introduced as a petty criminal and Angry Black Man, its easy to understand why people are upset at the change.
All of "Bart"'s Origin in the New 52.
They Wasted a Perfectly Good Plot: The TV episode when Barry was thrown into the future could have been the perfect time to introduce the costume holding ring into the series.
Unpopular Popular Character: Captain Boomerang. In-universe not even his fellow Rogues like him, but fans consider a memorable and likable villain.
Minor character Fallout grieves over the deaths of his wife and daughter, who died when he tried to get close to them upon first transforming into hisnuclearform.
Magenta can also qualify, as her mother believed that she was possessed by the devil, and the cause of the car crash that claimed her father and brother. She was pretty much pushed into heroics by her then boyfriend Wally West, and was later exploited and brainwashed twice, first by a doctor at STAR Labs who created her split personality as an assassin, and later by Raven during one of her jaunts on the dark side. Her fragile psyche and damaged mental health can be attributed to the times that she's fallen in with the Rogues, or any other villain the Flash has fought, such as the Cicada cult.
New 52 Wally. He's a twelve year old kid who was abandoned by his father when he was a baby, who's mother is missing and may even be dead, just had his Uncle Daniel (the only father figure he's had up to that point) carted off to prison by The Flash, and is now living with an Aunt Iris (that he's implied not to know very well) just so he won't be taken by Social Services. Spray painting the Flash Symbol and then putting an anti-sign (most likely to relieve both anger and stress) is completely understandable from his point of view. Given everything he's been through it's surprising that that's all he did. He also seems to feel at least a little guilty about causing his Aunt trouble.