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.
Maya. Fans didn't like her for being some random new love interest for Lucas who got way too much Character Shilling, despite the fact the viewers didn't know a thing about her, which led to them questioning why Lucas really cared that much for her. Nevertheless, Maya dying right beside him after he threw his entire life down the toilet just to be with her was pretty damn harsh.
No one really liked Tariq Masood for being a boring Replacement Scrappy for Malcolm. But watching as he's poisoned by some "vagrant" he bumps into, and then crying in the back of a taxi cab upon realizing this shortly before he collapses on the sidewalk and dies in front of Calum is a horrible and anticlimactic way to go. It certainly didn't help that he was finally starting to get Character Development in the episode he died in.
The first episode, "Looking After Our Own," has the far-right white supremacist Robert Osbourne. Planning a series of race riots to force Parliament to crack down on immigration, Osbourne also viciously abuses his wife Claire. When he realizes MI-5 is watching him, he has their contact murdered. Osbourne is revealed to be connected to a human smuggling operation so he can choke off the asylum centers- with the immigrants occasionally thrown into the ocean. When he catches the agents after him, Osbourne shows what a monster he is by torturing Helen. First he forces her hand into a deep fat fryer. Then her head.
Mohammed Rachid is a radical mullah who has taken over a Mosque in Birmingham England, from the kind and moderate Imam who founded the Mosque. Rachid hates all Westerners and uses the Mosque as a recruitment ground to convince disaffected Muslim teenagers to become suicide bombers. When Rachid discovers one of his young followers is a mole for MI-5, he has the young man beaten with sticks and thrown out a window. Rachid sees his suicide bombers as disposable pawns in his mission against the West and has no intention of endangering his own life for that mission. Rachid's ultimate target for his suicide bombers is a playground full of children.
Harsher in Hindsight: During one of Lucas' flashbacks where he was being held in a Russian prison, he tries to hang himself, presumably because he couldn't take the torture anymore. Skip to the penultimate episode of Series 9 and you'll discover the real reason why he attempted to kill himself—and you may even hate the fact that he didn't succeed.
Iron Woobie: Harry. Specifically; having friends and colleagues frequently die, repeatedly being rejected by Ruth, having certain close friends and trusted allies betray him (and in one case having to kill them - Series 9 episode 1). Yes, Peter Firth is the master of sad and tired expressions.
It Was His Sled: Helen gets her face dunked in a deep-fryer and then gets shot in the head early on in the show.
Paul Rhys' incredibly hammy performance as Alexis Meynell in "On the Brink." Especially during his "WHERE ARE ALL YOUR BAAAAAWWWWWLSSS?!" speech, and the part where he randomly snarls and shakes a handrail upon discovering that Ros works for MI-5.
The sixth episode of Series 8 involves a banker targeted by a conspiracy, which murders his girlfriend and her son when they break into their flat. The team keeps this secret from him until he learns the truth from a news report and makes the most hilarious face ever seen on television which undermines an otherwise very dramatic scene.
Ashur sobbing in episode 10.04 can come off as this since he sounded more like a dying animal moaning. Given the situation, it's Narm Charm for some.
Ben Kaplan wasn't much of a likeable character for being a Jerkass and a cliché journalist who only cared about his personal career. Come Series 7, he Took a Level in Badass and has become more fleshed out, and he took his job as an MI-5 agent more seriously.
Dimitri wasn't well-liked for being a boring agent randomly thrown into the show for the sake of having more characters. But after his Day in the Limelight in episode 10.03, fans got to know more about his personality, and his role in the show became much more important.
Calum is arguably the most hated MI-5 officer in the whole show for being a snideJerkass who whines at times and thinks his sarcastic behavior is actually funny. While he did become a bit nicer after Tariq's death, his snarky attitude still remained.
As mentioned on the main page, Juliet Shaw's Face-Heel Turn that put her in charge of series six's season-long antagonists, Yalta, who were staunchly anti-American to the point of supplying Iran with the components for nuclear weapons just to keep the US in check. Besides having been shown to have strong pro-American sympathies in series four, Shaw wasn't present in series six until the reveal in episode eight (having been Commuting on a Bus with handicap access since the series five premiere), which prevented any meaningful build to the moment. Didn't help that her paralysis was also retconned in this episode.
Invoked in Series 10 with the discovery that Sasha Gavrik is really Harry and Elena's son. It feels forced and seemingly out of place because it's supposed to be. In the end, it's revealed that Sasha isn't Harry's son at all; Elena lied her ass off just to make Harry think he was the father for thirty years. Manipulative Bitch indeed.
They Wasted a Perfectly Good Character: Lucas North was introduced in the first episode of series 7 pretending to be a double agent for the Russian Federation with his Russian ex-wife Elizaveta as his handler. All in all, an interesting and shady change of pace from relative boy scout Adam Carter. An early episode even revolved around Lucas and Elizaveta being murdered as traitors by Russia if MI-5 acted on intel he got from the Federation. His precarious life as a triple agent seemed like it would be a recurring plotline for him. It lasted five episodes, not even to the end of series 7, before it was dropped for good and he became basically the new Adam Carter. At least until series 9 happened....