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.
Anti-Climax Boss: The Omniscient Council of Vagueness that is Satyr was portrayed as an Ancient Conspiracy responsible for significant amounts of evil. Vintage served as nothing but their Unwitting Pawn and Dokuro described them as being nothing short of unstoppable without the aid of a mysterious extremely powerful devil. In the end, they serve only to harass the goddesses. When the heroes turn around to actually fight, they get defeated in all of a couple pages.
Shiori's Conquest Arc in the Anime, which lasted for three episodes, still had rather stretched out scenes, all of them from Shiori's perspective. The whole thing could've been reduced to two episodes.
The awakening of the final Goddess, Mercury, drags on throughout the entire second half of the Goddess Arc due to the Love Triangle mess between Keima, Ayumi, and Chihiro. By the time Keima was on the final route after realizing Mercury was in Ayumi, people just wanted the arc over and done with.
A similar case occurs with the "Third Memory Fragment" Arc, as most of the interesting stuff involved Keima and Kaori's clashes on world view, helped along with her extreme Freak Outs. After that though was the string of events that repetitively broke most readers' Holy Shit Quotient meters like they were glass figurines.
Ass Pull: FLAG 266's revelation that ELSIE is actually the reincarnated soul of a Weapon of Mass Destruction-slash-Eldritch Abomination, and also Satyr's ultimate weapon, is either this, or the most brilliant thing to ever come out of Wakaki's mind. Especially because it's implied she had at least some involvement in dismantling Satyr, at the very least being powerful enough and friendly enough to Keima and friends to stop time and explain things to him; it's a very short jump to assume she may very well have prevented the whole disaster by merely being the weapon to cause the disaster in the first place, leaving the goddesses' power unneeded.
Base Breaker: Chihiro, especially after the events of FLAGs 189, 267, and 268. Some fans argue that, being The Generic Girl, she is just too bland to be even involved in any way to Keima's story. Others would retort that such character of hers served as a proper contrast to Keima's Dating Sim-oriented idealism, being the character that rarely pops out in games, and the one person who, in Keima's own words, "can knock me down that far". (Just refer to the trope entries in this page where her name pops out.)
The contents of Keima's last letter to Tenri. While FLAG 263 has a major part of the letter revealed to the readers, the last bit was apparently left out, causing a lot of fans to speculate that the said portion of the letter would resolve the Ship-to-Ship Combat of the story by revealing who Keima actually likes. Comparisons to Laplace's Box and other similar items are abound. The final FLAG reveals what was not disclosed to the audience: a statement written by Keima addressed to Tenri where he reaffirms that he and Tenri will not end up together, thus leaving Tenri saddled with the knowledge that her love will be forever unrequited.
Not even counting the Ship-to-Ship Combat, the events of FLAG 268 itself caused a massive split in the fandom. People are either satisfied with how it ended, or are disappointed that it felt like a rushed conclusion, and left many plot-points unanswered. Some have even begun to develop their own Fan Fic that either explains the canon ending in greater detail, or changes it up entirely to end in a different way (including Keima's choice regarding his harem).
The anime adaptation of the Goddesses Arc. The other capture target arcs between it and the second season that had no relevance to it were completely ignored, while the ones that werenote Tsukiyo's, Yui's and Akari's got brief flashbacks in the first episode instead.
The first time was part-way through the Goddesses saga where people were sick, and tired, of dealing with the Keima/Ayumi/Chihiro Love Triangle, and just wanted to see the final Goddess actually get awakened so that the reader can finally witness the ending of the Goddess saga.
The second time was the Heart of Jupiter saga in its entirety with people wondering why the manga even needed to be continued after what looked to be a good cut-off point at FLAG 189. People ended up less invested reading arcs from this saga that just didn't seem that particularly interesting; going away from the "capture targets" method that people read the previous sagas for.
Despite having a short early arc and rarely having any further appearances, Kanon was the most popular character by FLAG 156. Elsie, by contrast, only manged sixth place.
Chihiro became one during the latter events of the Goddesses Arc, due to having been confirmed to have liked Keima before her original conquest had even occurred.
Lune, the New Devil antagonist of the Goddesses Arc, is this for many for being one of the few relatively evil characters seen so far in the manga, and having a refreshing personality to go with it. The anime producers caught on and changed it around to having Lune stabbing Kanon instead of Fiore to enhance her role in the arc.
Despite a rough introduction, Urara quickly became one due to her...naughtiness as a minor. Then later on, a Weiss enters her and she suddenly ages into a fine looking woman.
Nikaido was always an incredibly popular character as Keima's Hot Teacher with people wishing for the day that she becomes one of Keima's capture targets. However, once the revelation came about that she was Dokuro from the Heart of Jupiter saga, a character that Keima has kissed on more than one occasion, her fanbase skyrocketed.
Harsher in Hindsight: During Chihiro's capture arc, Keima said that there is a weird factor about her— "there is a boy she likes"—Elsie also speculated that "crevice in her heart got bigger because of her love". Everyone forgot about it after the capture, and then in FLAG 166 we got to know that the boy she liked is Keima, and it was her inability to cope with this (with the fact that she likes probably the least popular boy in the whole school) that made it possible for a Weiss to possess her.
In FLAG.183, Keima proclaimed that the Marriage Ending ("an ending among endings" ...in games) must be at the very least destined since childhood. It's later revealed that thanks to Mental Time Travel shenanigans around ten years prior to that point, Keima in his younger self body had to make sure that all the goddesses hosts would become capture targets, hence "destining them since childhood" to meet and be saved by him.
Urara's introduction has her trying to act like an adult. A few FLAGs later, she turns into an adult.
Remember the early FLAGs where Nikaido kept on taking Keima's PFPs and beating him for playing dating sims in class? Yup, it's all because she was jealous that her "onii-chan" was paying attention to the girls, in games and later out of games too, and not to her.
Iron Woobie: Keima. Though he says he doesn't like real "girls", it becomes apparent that what he hates is in fact everything real—he finds himself at such odds with the real world that he actively rejects it and takes refuge in games. What he's doing is going outside an incredibly small and restrictive comfort zone in order to do something he would run away to avoid if he had the choice, but he's shouldering the burden and putting his everything into this thing he hates doing. And, though he might moan, he never runs away, never breaks down - he always keeps his cool and goes through with it.
"I can see the ending!" Hear the line anywhere, and the first thing that comes to mind will always be Keima.
"Game Over". The orb breaks, the world becomes blank except for the goddess hosts. There have been many other The End of the World as We Know It jokes concerning FLAG 260, but this one is agreed to be the best fit for this scenario.
Moe: The girls that Keima has to court are plays on Moe stereotypes in varying degrees. Being a Genre Savvy Otaku, he goes into a rant when he encounters one that doesn't seem to fit with the archetype.
Nightmare Fuel Station Attendant: Elsie simply doesn't comprehend why humans might be disturbed by her cooking, or the souvenirs she brings back from hell, or the idea of eating something alive because it's fresher.
KeimaŚElsie, though the popularity of this pairing waned after the first couple arcs. Being Put on a Bus during the Goddess arc didn't help, especially as it allowed Keima to interact with many of the other female characters of the story while she was gone.
KeimaŚHaqua, due to being one of the only girls who knows Keima, rather than a persona he's putting forth for a capture.
KeimaŚDiana as a bonus for Tenri fans. Diana openly admits this in FLAG 167 (appropriately titled WHAM), where she realizes that her power has been limited by her own guilt over being attracted to Keima.
KeimaŚYui gained ground fast. Yui is pretty much the only girl that has actually made Keima flustered, and he admitted that even though he was trying to just put on an act, he wound up letting her take the lead a lot and she made him feel "fuzzy headed". Haqua even notices and comments on it, teasing Keima that maybe his "real" girl type is one with a forceful personality. Not to mention the fact that she's the first human girl he's seen naked because they swapped bodies for a time. Actually, the reverse holds true as well.
There's also a lot of support for a KeimaŚKanon pairing, especially in the Goddess Arc where Kanon is the first girl to confess to him, and was the catalyst for a big chunk of Keima's Character Development.
The KeimaŚChihiro pairing got more fans supporting it thanks to the last few events of the Goddesses arc, as fueled by the revelation that Chihiro had probably genuinely loved Keima before her capture arc, absent any manipulations on his part. Add the fact that Keima was forced to dump her (possibly out of confusion as to what to do next), and the shippers decided that he must end up with her as a way of redeeming themselves and everyone else from the backlash.
Surprisingly, KeimaŚAyumi is gaining ground as well, being Keima's very first capture target, and the last Goddess host to be revealed, and due to his (much-debated) declaration to her as his "First and Final Heroine". Also, she appears in the third memory fragment of the Heart of Jupiter arc, along with Chihiro.
There's also a slow-but-steady rise in support for KeimaŚNikaido/Dokurou as soon as FLAG 250 was released. (For security purposes though, this sub-entry will be a massive spoiler entry): She was chronologically Keima's first kiss; she was one of those persons who, like Tenri, was all the more supportive of his actions (even calling him onii-chan while Keima's in his 7-year-old self); and apparently, her identity was one of the biggest surprises of the Heart of Jupiter Arc, having been conveniently concealed (even from the readers) from the time she was revealed to have an involvement with New Hell.
In Kanon's first arc, she wasn't well-liked due to her Attention Whore / Yandere tendencies, but in her later appearances she's a much more endearing and likeable character, even turning into something of an Ensemble Darkhorse.
Breakout Character: The ending result. She now stars her own spinoff, Magical☆Star Kanon 100%.
Urara was disliked in the first few events of the Heart of Jupiter Arc for being nothing more then an annoying Bratty Half-Pint trying to act older then her age, but eventually, people warmed up to her once she started getting along with Keima, and learning of her back-story as to why she acts the way she does.
Pick the woman among Keima's capture targets (or someone else) that has the best chance to end up with him and let the debating begin. Tenri and Chihiro led the way for majority of the story's run due to being the only girls that actually liked Keima before he got involved in their conquests. Elsie, Haqua, Diana and the other Sister holders are also worthy choices among the fan base.
They Changed It, Now It Sucks: This type of reaction was bound to happen for some fans the moment it was announced that the 3rd anime season was going to be adapting the Goddess arc; effectively skipping the remaining conquest arcs that occur after Jun's conquest entirely with the exception of Tenri's arc which was done in prior OVAs to hype the upcoming anime adaptation of said Goddess arc. The first episode of the season airs, and most fans are, predictably, disappointed that they won't get to see the final conquests animated. However, on a brighter note, the first few minutes recapped the skipped conquests with most of the footage being from Tsukiyo and Yui's arcs due to their importance later on; showing that the skipped material is at least acknowledged to exist and that the anime is staying true to the manga...with the exception of Fiore being adapted out. What also didn't help was that the Goddess Arc and the Mai-High Festival Arc add up to 76 chapters, considered by many to be way too much for a 12 episode season. For comparison, the first 2 seasons covered 43 chapters.
Viewer Gender Confusion: Dokuro Skull, revealed to be female (like all other New Devils) in FLAG 151. The anime missed the memo; her Japanese voice actress uses such a gruff voice she ounds male, and her dub VA is male.
What Happened to the Mouse?: The apparently rushed ending seems to leave a very significant number of questions unanswered and plot threads unresolved; in particular Lune's whereabouts, Keima's still Disappeared Dad, and more importantly how Satyr was able to use Elsie for their plans.
With added Freeze-Frame Bonus: this page on FLAG 263, particularly the part of Ayumi's profile which mentions something about Chihiro's Notes. Even after the end of the story, the said part was not revealed, but was nonetheless brushed aside due to the events of the final two FLAGs.