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.
Foe Yay: Balthazar and Lucifer towards Constantine. He's not liking all the hellish attention he gets.
Ho Yay: Again, Balthazar and Lucifer towards Constantine
Meta Casting: Keanu Reeves was a very... odd choice to play John Constantine, but as it says elsewhere, the movie is good if you don't mind its In Name Only connection to the comic book. So as long as the protagonist doesn't have to be a fast-talking scouse Anti-Hero... who better for the role than Fli/mTheDevilsAdvocate?
Mis-blamed: Connected to the above, Reeves is blamed for pretty much every alteration between the movie and the original source material, apparently solely based on the rumors that he couldn't do the proper accent. Casting Reeves was apparently one of the last things done for the movie, after all the other changes had already been decided on.
Nausea Fuel: Satan sticking his hands in Constantine's chest and ripping out cancer tissue.
The novelization is even worse, as it goes into some of the torments in-depth, such as the buildings being built from bricks that are actually damned souls (many of whom are inquisitors from the Spanish Inquisition among them), and describes Angela looking into the mind of the Scavenger and seeing herself being raped by him (Mammon had promised him Angela as a sex toy if he complied with his wishes).
Visual Effects of Awesome: This movie has the coolest looking adaptation of Hell seen in cinema thus far. And the demon made of insects looks particularly awesome as well.
Special mention goes to the scene of Angela getting dragged through the building by an invisible force. This troper remembers sitting in the theater when the trailer premiered, jaw on the floor, and agreeing with her father that she was sure as hell going to see that movie.
What an Idiot: Balthazar doesn't realize that you can't be forgiven of your sins unwillingly. Then again, he's implied to have been born in Hell and they probably don't teach theology down there.
As previously mentioned under other tropes on this page, if you ignore the connection between the movie and Hellblazer, the casting choices are not so bad.
From the series:
Alternate Character Interpretation: In "The Saint of Last Resorts," is John being honest with Anne Marie when he gives her the spiel about how no cost is too high to protect the innocent, especially if the innocent is a baby, or is he talking out of his ass because he knows she'll fall for it and do what he wants? Later on, is Anne Marie being honest when she repeats it, or is she just throwing it back in Constantine's face to add insult to injury when she shoots him to protect the children from the Invunche?
Broken Base: The show's getting a rather nasty one after it was revealed that John would not be bisexual in the show, as the creators deemed it unnecessary to his character. Unfortunately, they also added "And besides, he sleeps with mostly women anyways." While there are some who don't care (citing how John's romantic partners tend to die in the comics, how he only ever slept with one man in the comics, or that they're gonna judge the show when it comes out and not before), a lot of viewers (particularly people who are LGBT themselves, or sympahetic/supportive towards such) are upset at the bi erasure. It doesn't help that bisexuality in particular, especially male bisexuals, are largely ignored in media depictions, and that LGBT characters in comic books rarely get adaptations, especially ones that retain their LGBT status if they do appear in adaptations.
David Goyer later said "We never said he wasn't bisexual," so this may change in time.
Cult Classic: The show has a solid following, but apparently it's not solid enough for NBC to make a concrete decision about whether to renew the show for a second season or cancel it.
Ensemble Darkhorse: Anne Marie managed to win over a lot of fans despite her short appearance. This may have to do with the fact she's one of the more moral characters on the show, able to stand up to John, has a strong link with his character, and is played by the very lovely Claire van der Bloom.
"Funny Aneurysm" Moment: The reluctance of Constantine to call himself a "Master of the Dark Arts" at first seems like a Running Gag, but the end of the pilot episode shows there is a very serious reason: his arrogance in assuming he was this and summoning a very powerful demon to take a lesser demon away, rather than exorcising said lesser demon instead, showed that he was, in his own mind, not worthy of calling himself a "Master", but rather the "Petty Dabbler" he calls himself in the show.
Growing the Beard: Audiences and Critics are like thought the first few episodes to be So Okay, It's Average, but showing potential. But audiences and critics alike both agree that episode four, "A Feast For Friends", which introduces more of Constantine's moral problems and backstory, is when the show really started picking up steam.
Older Than They Think: Some fans unfamiliar with the comics think Papa Midnite's "Ace of Winchesters" is a reference to Supernatural. In fact it is an artifact created in the comics, specifically as the main plot device between the Hellblazer/Hitman crossover Garth Ennis wrote.
The Scrappy: When the pilot was leaked, multiple critics online didn't like Liv, which helped cause her being written out. The AV Club and IGN both felt that Liv's departure was ultimately for the better.
Besides the above issue, some fans are also up in arms because the pilot mispronounces his name. The I sound towards the end if supposed to be long vowel, like "tyne" but they pronounce it "teen".
Also, NBC's restriction against explicit smoking though John is famously a chain-smoker. The pilot tries to work around it like by showing only cigarettes being stubbed out.
NBC seems to have lightened up when it comes to this, as several episodes have explicitly shown John smoking.
In a strange turn of events, some fans are taking umbrage with the fact that the show is taking more cues from the source material than the film. A case of 'They changed it back, now it sucks.'
They Wasted a Perfectly Good Character: It's obvious that the decision of Liv not staying on the show was made very late in production of the pilot, with all the backstory built up for her going to waste. The plot of at least the first season still depends upon her involvement as well, since Constantine is following the markings she left behind on the map. The producers' vagueness about why Liv was cut isn't helping.