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.
The Comic Book
Complete Monster: The Purple Man, real name Zebediah Kilgrave's power is mind control. How he uses it is utterly despicable. Revamped into an amoral sociopath, Kilgrave defeated the young superheroine Jessica Jones and made her his mental slave, forcing her to watch him commit crimes and assist him, while he heaped physical, verbal and mental abuse on her, but stopped short of outright rape because she 'wasn't worthy' of him. Not content with humiliating her, he forced her to watch him do things like mentally order people to stop breathing when it annoyed him. He forced Jessica to stand at his bedside and watch as he used his powers to date rape women, helpless to do anything about it. Kilgrave is what happens when you give someone who thinks It's All About Me is a code to live by, remove any empathy for other beings, and give them mind control powers and a serious desire to hurt others.
The TV Series
Alas, Poor Scrappy: The shot of the dead Francie, after the double takes over is heartbreaking.
Ensemble Darkhorse: Sark was supposed to be a one-off character, a gofer for K-Directorate or Irina or the Alliance. Fan reaction to David Anders was so great that he became more and more high-profile, eventually taking over the Big Bad mantle in the series finale flash-forward.
The end of S3/start of S4 was the sharkjump. They show had gone off the rails because of the utter failure of Abrams and Co. to cope with the absence of Lena Olin (not her fault, theirs), and they tried to reboot the show, more ot less recreating season one with a new SD-6 under Arvin Sloane. The broke continuity, rewrote Rambaldi's prophecies, scrambled characterizations around, and stumbled around in search of a plot. This whole thing was SUCH a huge insult to WSOD that the writers often Lampshaded it themselves, especially through the rational, accurate assessment of the Arvin Cloane character, who points out aloud the sheer absurdity of the idea that Sydney and Dixon would be willing to work with Arvin Sloane at all after the things he'd done to their loved ones.
A large amount of people would say Season 3 was the jump point, but after Season 5 it definitely got worse.
Things went off the rails in Season Three, but not irreparably. The show could still have recovered if the producers and writers had simply done the rational thing and written Irina out of the show. Granted her fan base would have howled, but the absence of Lena Olin meant they could not be satisfied anyway.
Many fans blamed this on JJ Abrams shifting his focus to Lost, which premiered around the beginning of Alias's 4th season.
I don't buy it. The problems had already been brewing from the start of S3, though it could have been fixed up until the last few eps of S3. They utterly bungled the absence of Lena Olin, and they made the error of trying to make all their fanatic raving fan factions happy at once, and ended up infuriating them all.
Really, the base of this all is the last scene of S2. The time jump was a cool idea in the short term, but it brought too many changes to the characters and setting that the writers were unable to negotiate over the long term.
Moral Event Horizon: Sloan crosses it in the first episode by ordering the murder of Sydney's fiance, but keeps going. Killing Dixon's family in retaliation for Dixon accidentally killing Emily is another. Accidentally throwing Nadia through a glass table and killing her is a third.
Narm: Vaughn trapped in the compound in Taipei, during "Almost Thirty Years", complete with slo-mo drowning, and yelling at Sydney to leave.
One-Scene Wonder: The show had many, many memorable guest stars.Quentin Tarantino, visibly enjoying himself as McKenas Cole. In Season 1 he's more of a Two Episode Wonder, but when he pops up again in Season 3 he fits the trope perfectly.