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.
The Shiori Arc, 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.
Shiori is apparently very popular among the Japanese. In fact, her first capture arc really pushed this series' popularity to be on the map. Considering her long absence, she ranked number 3 in the most recent official poll. She was just right behind the neck to neck contenders for number one spot Tenri and Haqua, both appear in many more chapters compared to Shiori (at least until it became rigged by 2channers after the first few days).
Despite having a short early arc and rarely having any further appearances, Kanon was the most popular character by chapter 156. Elsie, by contrast, only manged sixth place.
Judging by the later chapters, Chihiro is on her way to becoming one. After all she has recently been confirmed to have liked Keima before her original conquest had even occurred.
Lune, the New Devil antagonist of the Goddess 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...naughtyness as a minor. Then later on, a Weiss enters her and she suddenly ages into a fine looking woman.
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 chapter 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.
Urara's introduction has her trying to act like an adult. A few chapters later, she turns into an adult.
Ho Yay: Sort of, in Chapter 124. Yui, in her masculine clothing apparently trying to seduce Keima. Made worse by Keima's rather feminine response probably due to spending time in Yui's body.
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.
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 chapters 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.
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 her introductory chapters 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 amongst 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 lead the way at the moment 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 amongst the fanbase.
And if one doesn't side with any of the ladies...there's always Yokkyun.
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.
Viewer Gender Confusion: Dokuro Skull, revealed to be female (like all other New Devils) in Chapter 151. The anime missed the memo; her Japanese voice actress uses such a gruff voice she sounds male, and her dub VA is male.