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.
Complete Monster - Averted for the most part as many criminals were portrayed as too stupid to understand the depths of their actions. Even mass-murderers such as Pamela Wilgis and the snipers (the original and his Jack the Ripoff) were portrayed as insane or pathetic. On the other hand, Luther Mahoney was a monster: a smooth, intelligent, charismatic man who could have done anything he wanted in life and instead chose to be drug lord who had children killed over a few hundred dollars and ordered a local corner store owner murdered simply because he chased away the dealers who would sell outside his store.
His nephew Junior Bunk also qualifies. In the course of a few episodes, he murders witnesses, kills a judge and shoots up the Homicide unit (Killing three officers and wounding two detectives) with a joyful smile on his face. His mother Georgia Rae isn't any better, waging a psychological war on the Homicide unit, taking over the drug deals and using her son to get revenge for her brother's death.
Creator's Pet — Falsone, who solved all of his cases (including one cold case), made up with his wife, and simultaneously successfully investigated every single aspect of Kellerman's Mahoney shooting without attracting any criticism outside of Kellerman. He was brought closer to Earth in Season 7.
Arguably Ballard was a far worse offender, at least initially. The episode in which she first appeared had Giardello and all the members of the unit who didn't get transferred talking about how great a detective she was. The same three-parter had her moaning about how Pembleton doesn't seem to respect her and has Pembleton refuse to follow a completely obvious lead (instead following one that was completely out there) just so that she could look good. Fortunately she became increasingly side-lined (unfortunately in favour of Falsone). Unfortunately she never went away.
Most of Season 6's additions probably also count, with fan-favourites Pembleton, Kellerman and Lewis getting sidelined in favour of storylines involving the newer characters. The TV-movie saw some of these characters get relegated for old favourites.
Pembleton was, and continues to be, Homicide's most popular character. To the point that the only season without him is what killed the series.
Gary Stu — Falsone, especially in the sixth season.
Harsher in Hindsight — In "A Many Splendored Thing" Adrienne Shelly plays the friend of a murder victim who thanks Bayliss and says she is happy to know that if she is murdered, Bayliss will avenge her. In 2006, she was murdered in a robbery of her NYC office.
The first episode of season six features a murder committed by a member of a reputable black family. They give the idiot ball to Pembleton, who refuses to even consider that a member of this family would perpetrate this crime because they were black, friends of Gee, and did a lot of good for the city, even though their only other lead was such a long shot and Pembleton is, unlike Bayliss, usually free of such hang-ups. The reason they gave Pembleton the idiot ball was so Ballard could look good. Understandably this probably pissed off a lot of fans who hated Ballard, and there were quite a few.
Andre Braugher, who played Pembleton, was also probably very irked about it as well, as after the season ended, he wound up leaving the show. To be fair, the network had already scuttled the Pembleton stroke arc (which Braugher liked because he wanted to depict a Pembleton who had to face more challenge and difficulty in his work) and now he had to contend with being side-lined in favour of the network insisted super-model cop.
Jerkass Woobie: Munch. An insufferable, sarcastic jerk he may be at times but so many bad things have happened to the guy (Being dumped by various women, being bullied in school, being clearly traumatized after seeing his friends shot, and feeling guilty over his abusive father's suicide) that You can't help but feel bad for him.
Pembleton could be a complete jerk but only a person with heart of stone couldn't feel sorry for him after his stroke. Pembleton's best resource was his mind, so his loss of cognitive function is tragic, added to which his wife just had a baby, making his inability to go and work as well as his need to be cared for by her even more tragic.
Felton, again a complete prick but given the way his crazy wife treated him you couldn't help feel sorry for him when he started drinking, turning up to work late (or not at all) and losing key pieces of evidence. And while you kind of agreed (Felton was nowhere near the best detective), Gee telling him that he wasn't good enough for the unit after he'd been shot must've hurt.
Bolander could be a cantankerous old man, especially in the way that he treated Munch. But you felt sorry for the way his wife left him, especially since they obviously still loved each and especially since Bolander finds it so difficult to try and rebuild his life outside of work after losing her.
Kellerman. His wife cheated on him. His partner (and supposed friend) won't open up to him. He's accused of taking bribes even though he didn't. He has a somewhat abortive relationship with Julianna Cox which basically ends because he's such a mess he can't explicate his feelings for her. He kills a bastard drug dealer pretty much in defence of his supposed partner/friend (how was it going to look if Mahoney revealed Lewis beat him up? Plus, he lowered his gun but he didn't drop it) and what does Lewis do in response? Turn his back on him. He eventually gets kicked off of the Police force for killing a total bastard, and everyone he worked with thinks he's trash.
Moral Event Horizon — An interesting example of this trope involves Kellerman. His point of no return does not seem to be the execution of Luther Mahoney, but rather a smaller, quieter scene in which he laughs at a young dead drug dealer on a crime scene, arriving as far as posing for a mock picture next to his corpse. From that point on, the writers portrayed him as increasingly bitter and unlikable - no longer an Anti-Hero, but simply a Jerk Ass.
Det Ballard: We speak for the dead, remember? Det Kellerman: Screw the dead. What have their moldering asses ever done for me?
The latter scene was facilitated by the former, which was such a moral event horizon that Kellerman, who previously quit, started smoking again and descended into bitterness. He somewhat redeemed himself during his guest appearance in season 7, however.
Nightmare Fuel — The episode "Subway." A man (Vincent D'Onofrio) is shoved on a subway platform and is pinched at the waist between the platform and a subway train. We spend the rest of the episode with him as he's waiting around to die. He is lucid the entire time.
Poor Man's Substitute: The writers seemingly wanted to set-up Gharty as Bolander's poor man's substitute. This is lampshaded by Munch, who claims that a certain trait of Gharty's reminds him of Bolander. However, this analogy falls down in a number of places. For example, while broadly they might be similar (Both are older, in less than perfect shape and fairly conservative Gharty has a number of negative personality traits not shared by Bolander that ultimately make him less likeable (Gharty is a bigot, a coward and usually flirts with Munch's bartender/girlfriend/fiancée/wife). In addition Gharty lets himself get pushed around in ways that Bolander would never stand for.
Replacement Scrappy — Ballard, Sheppard, the younger Giardello, Falsone. Any character introduced after S5.
Season 7's line-up is this particularly because of Pembleton's departure. While Giardello Jr is a decent character (albeit underdeveloped), he is no replacement for Frank Pembleton.
Kellerman at least partially qualifies, being a younger, more attractive replacement for three older, fatter detectives. But unlike previously named examples, Kellerman fit perfectly into the cast and rhythm of the series.