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.
Alternate Character Interpretation: Danny, being a major example of the Cowboy Cop trope, is subject to this a lot within the fandom. The show seems to encourage this, since some of the showrunners worked on The Sopranos, and probably wanted to include at least one morally grey character. Basically, opinion on Danny is divided between those who approve of his policing methods and those who find them excessive:
Those who agree with Danny feel that his willingness to break procedural rules is justified given what he goes up against, and at worst view his harsh actions against as merely an example of Pay Evil unto Evil
Those who disagree with Danny's methods often view him as dangerous, reckless, and view him as having a disturbing disregard for New Yorkers' civil liberties, and many view him as being a Knight Templar who often comes unnervingly close to becoming as bad as the people he fights. One reviewer pointed out that when Danny decides that a suspect is guilty, he goes after them with reckless and extremely aggressive tactics...but what happens if he ever mistakenly becomes convinced that an innocent person is guilty? Not to mention the dubious legality of some of Danny's methods.
Given that Grandpa Henry virtually always (loudly and vigorously) agrees with Danny during debates at the dinner table, opinions on Grandpa Henry have lately also become divided along the exact same lines, with Danny's supporters admiring Henry's opinions, and Danny's detractors disagreeing with them.
As of Season Five, the show does seems to be much more aware of the Unfortunate Implications caused by Danny's methods (and Henry's support of these methods), and there have now been several episodes where various characters call Danny out for his tactics, and where Danny and Henry (in separate episodes) wind up in hot water for their Cowboy Cop tendencies/views. Even Frank and other family characters have now given Danny and Henry a What the Hell, Hero? speech.
Dick Reed, the serial killer/rapist from season 1's "Re-Do," is a thoroughly misogynisticSmug Snake who gets his conviction overturned because a lab tech screwed up the protocols. He scares his surviving victim into silence and lets one of his fanboys have her. He also beats his sister and ties her up for saying something she "shouldn't have." He shows zero remorse and no redeemable qualities and after slitting a janitor's throat, he finally attempts to perform his usual MO on Erin Reagan, the ADA who convicted him, before she is saved by Frank.
The Phantom, aka Donald Washington, from season 2's "The Life We Chose," is a cold-blooded crack dealer and murderer. He shoots detective Cruz and kills Detective Gates, who pleads that he has three kids, to which Phantom replies "Too bad for them". This shocks his cohort, who was only planning on robbing them. Phantom later kills his cohort, targets Ray Bell, a man who ratted him out to the police, and holds Ray's family hostage.
Yuri Denko, an arms dealer whom Erin prosecutes in season 2's "Working Girls," is a cold-blooded sadist who has no problem executing a man's wife in front of him in their living room over a business dispute. He then threatens the man's children in a courtroom outburst to scare him into not testifying, then has him killed after the threats scare him into running. He puts out a hit on the surviving witness, hounding her mercilessly, and went after her grandma back in Russia, too (the FSB got there first and protected her).
Critical Research Failure: At the beginning of "Hall of Mirrors", there is a cricket game being played in Central Park with remarkable accuracy, except that there aren't any wickets. It's funny as hell.