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.
Dueling Games: Played straight it would be Dream C Club. However within the franchise itself Cinderella Girls and Million Live are fueling a small rivalry. Some gamers are greater fans of one over the other, and upset Namco Bandai divides attention between the two properties for equal exposure... it is claimed both lack a dedicated anime or console game because of this. Those who enjoy the original 765 Pro characters and care nothing for the spinoffs, make the same claim for CG and ML taking away attention from the original series. On the brightside all three properties are massive successes and the Namco Bandai attention division only delays things. Eventually there will be a Cinderella Girls anime and console (or handheld) game, multiple hints keep pointing at it, and several Million Live idols are in the iM@S movie as guest characters.
Ear Worm: So, so many of the songs. Especially if you know the dances that go with them.
When news of Idolm@ster DS first emerged, Ryo was generally considered the least interesting character. Cue the reveal of one minor detail, and Ryo near-singlehandedly doubles the game's preorders.
From the Cinderella Girls, the starter idols for each category, Uzuki/Rin/Mio, could be considered this, as despite being the face of their categories, since they are Com Mons, one would expect them to be overshadowed easily. Instead, Rin consistently places high in popularity polls, even taking first place in 2014's election, and while it took a while for Uzuki and Mio's popularity to reach Rin's level, they still finished at a respectful fourth and fifth place in the same poll. Both Uzuki and Mio's popularity came about from their underdog status, and Uzuki is even (possibly) the main character/focus for the upcoming Cinderella Girls anime.
Any of the CG Masters could be considered this, but special note goes to:
Other CG master idols of note include: The Jougasaki sisters, moreso with Rika than Mika; Anzu Futaba and Kirari Moroboshi, due to their juxtaposition and Ascended Fanon pairing; Nao Kamiya and Karen Houjou, Rin's classmates and friends who are frequently shown together; Koume Shirisaka, for her Shrinking Violet personality; Mayu Sakuma, for her Yandere traits; Syoko Hoshi, for her Ax-Craziness.
In an odd subversion, some idols who don't place anywhere in the top 50 still receive voices and CDs, giving them more attention. Sae Kobayakawa and Yuko Hori from the most recent batch of releases fall under this category.
While she doesn't have a voice or song, Shizuku Oikawa deserves mention for TWO BIG reasons.
Fandom Rivalry: It might not be reciprocated, but because Kentaro Miura has been known to be a huge fan of this series to the point of schedule slipping, Berserk fans have been shown to dislike this series because of the competition; it isn't unusual to find comments in Idolmaster-related YouTube videos or web-page articles commenting on Berserk potentially being delayed again and/or general reactions of "He put Berserk on hiatus for THIS?!".
Fan Nickname: "Broducer" for the anime version of the Producer.
Fanon: Many fans consider Clarice from the Cinderella Girls series to be blind because all of her cards have her with her eyes closed.
Most of the girls' backstories and personalities are suggested by fandom, actually. An example of this is big sister-like Minami Nitta suddenly becoming a yandere.
* Fan Translation - As of November 2012, The Idolm@ster SP Perfect Sun is fully playable in English. Chihaya's route from Missing Moon is also released with Iori and Yukiho from Wandering Star. The people at TLWiki are continuing to work on the other two PSP game routes.
Hilarious in Hindsight: The two fake trailer movies that star the entire idol group now that there is an official movie.
Iori has covered Nadeko Sengoku'sImage Song, "Ren'ai Circulation". It turns out that, like Iori, Nadeko isn't as cutesy or innocent as she appears to be.
Yao Fuei Fuei's base card is notoriously common, and due to its extremely low stats, cost and lack of a Skill is good for little more than vendor junk. Fans have taken to referring to a Fuei Fuei draw as FFDY.
There is the てってってー (Te Te Tee) meme from Nico Video, where remixes of alldifferentsorts are done with a BGM song from THE IDOLM@STER, as well as the popular trend of putting the choreography of the dances (Usually the most popular ones.) to different songs.
Due to the popularity of iDOLM@STER in Japan, Sony offered a Haruka "skin" for avatars in Playstation Home. The resultant skin, really intended like a plastic mask for the avatar, fell so forcefully into the Uncanny Valley that "Home Haruka-san" (as she's been dubbed) has become a minor meme on Nico Nico, particularly in the Vocaloid fandom where the 3D choreography program "Miku Miku Dance" is readily available. Home Haruka-san should be treated with the same amount of fear and respect as you would give Michael Myers.
Yukiho's Catch Phrase ("I'll go bury myself!") soon become a meme about Yukiho's extraordinary ability to dig holes on every surface, as well as the ability to summon shovels from Hammer Space. Somehow ascended in episode 15 of the anime, where she (as a super robot) has drills and an obsession to dig through stuff.
Kuroi also got one thanks to his nonsensical sense of board game: "CHECKMATE!"
Narm: In the anime, Kuroi seems to have a fetish on chess, Go, etc and likes making up things to make Check Mate (and says it out loud)... until you realize that he's just arranging the pieces into some shapes he likes rather than making an actual check mate situation... Clearly the man knows nothing about the board games he plays.
No Export for You - While the console games at least require the Japan region consoles, THE iDOLM@STER Mobile i takes it to the next level. Unless you have your iPhone "modified", you have to live in Japan in order to play it.
Finally averted with the Puchim@s anime and the Shiny Festa games. The former was simulcast by Funimation and the latter received translated ports on the iOS App Store.
Rescued from the Scrappy Heap: While Jupiter wasn't well received during their debut due to the series consisting of female idols up until that point (except for Ryo) , fans have somewhat warmed up to them in the anime version; while they're still high-and-mighty jackasses, they clearly show disgust when they realize Kuroi has been skipping Moral Event Horizon behind their ears.
Ship-to-Ship Combat: Makoto is most commonly paired with either Miki or Yukiho. They are depicted as rivals in many MADs.
Tastes Like Diabetes: It's a game about J Pop, so quite a few of the songs. The spinoff manga "Puchimasu!" probably qualifies too.
The preschoolers in Episode 24.
Shiny FestaVisionary is two minutes of sugary cuteness.
Ugly Cute: Harukasan. With her constant wide mouth and eyes, she like the pug of the Petit Idol gang.
The Woobie: Poor, poor Chihaya-chan. Not only her past is that dark and troubled (for this kind of series' standards anyway), the last moments of anime episode 19 will make you want to hug her... while cursing at Kuroi and scream for his death for what he did to her.
Haruka becomes this in episode 23, since all of her friends are busy and have no time for her, a show that was suppose to happen gets canceled, and Miki starts viewing Haruka as a rival. The ending makes it worse.