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.
Is Mato just your typical na´ve girl, or is she in denial and being forced to regain courage? Is Yomi a kind, sweet girl with the occasional disturbing thought, or is she a psychotic Yandere? And is Yuu just a Genki Girl who feels left out nowadays or is she the Chessmaster like other fans are considering?
Is the Otherworld just symbolic? Are its denizens awakened shards of the characters' psyche? Separate spirits that played Grand Theft Me and/or The Symbiote to the schoolgirls?
Les Yay: There is quite a lot - Mato even tells Yomi "I love you!" at one point (she's joking... perhaps). In fact, Les Yay is the driving point of the story, if you consider their close attraction love. It's definitely a Romantic Two-Girl Friendship.
Even though Black★Rock Shooter came first, comparisons with Puella Magi Madoka Magica were perhaps inevitable. The original OVA already had comparisons made to the Personaseries, which are further amplified here.
Yuu even asks Mato to say Black★Rock Shooter isn't her.
Kagari is voiced by Eri Kitamura, who also voiced Sayaka in Madoka Magica. Like Sayaka she has relationship issues, questionable mental stability, and an alternate persona associated with wheels. Amusingly, she ends up more like Mami Tomoe from the same series — Chariot, Kagari's other self who appears prominently in the promotional material, gets her head cut off in episode 2. It helps that Kagari and Chariot resemble Mami in other ways, such as theme colours and having issues with friends and loneliness.
Die for Our Ship: Since Yuu's the main one getting in the way between Mato and Yomi, Mato/Yomi fans do not like her "interference". Even Yomi seems to feel this herself.
With Yuu getting a MAJOR Adaptational Badass in the anime, expect to see some Mato/Yuu shippers bashing Yomi.
The same thing happens with Kagari, with how she basically had Yomi on a leash. She made things even worse for herself when she tells Mato straight to her face that Yomi's heart is dead. She gets a nasty slap in the face for it.
The website takes this Up to Eleven, ranting about their "bonds" together and even dropping the word "love" semi-vaguely in there.
And of course the anime just completely runs with it in the first episode with Yomi calling Mato "beautiful".
The second episode has Yomi's mother refer to Kagari as having always felt "love" for Yomi since childhood. Wonder what kind of love that is...
This exchange should pretty much sum it up:
Yuu: "Hey Mato, you want a boyfriend?"
Mato: (incredulous look) "Eh? Too soon!"
Yuu: (slyly) "I guess you have your Tiny Bird."
Mato: (flustered) "Don't make assumptions!"
Not to mention the whole "Mato and Yomi blushing at calling each others' first names", Mato and Yuu apparently the only ones to share a bed at the training camp, and the part where Yuu points out that Mato's been with her "Tiny Bird", Mato reacts all giddy and embarrassed.
In episode 3, Kohata slaps Mato on the butt. As Mato leaves embarrassed, Kohata says she is cute.
Mato and Yuu may have possibly the sweetest moment in the anime (Episode 5), to the point where they almost kiss in the middle of a sunset.
Spiritual Licensee: The show is actually quite compatible with the Persona series. It features a tarot motif, the viewpoint switches between high school relationships and a different dimension where shadowy counterparts of the characters fight one another, and friendship is a very prevalent theme.
They Changed It, Now It Sucks: The characters were redesigned between the OVA and the Anime, particularly Yomi and Dead Master. Some fans were displeased.
The Woobie: Strength, who, after developing emotions, realized the impact of the pain she was taking on from Yuu, was suddenly catapulted by her human counterpart to switch places with her and forced into the real world, had to adjust to the strange new world with no friends until Mato's appearance, and generally had to cope with not really existing to people. She only wants to help Mato and the other girls, but quickly ends up losing Mato to (Insane) Black★Rock Shooter, and when trying to save her and Yuu from a potentially horrible fate, ends up scorned by Yuu herself.
Narm Charm: The first hour of the game runs on this, with the PSS best described as a Ragtag Bunch of Misfits who are desperate for hot chicks. Once the Draco crashed, however...
That One Level: The 3 motorbike segments in New York can easily become this if you don't get the hang of things. And there are Records which require a No Damage Run for all three segments.
Several of the Extra Missions can be this as well since many of them have instant game over conditions ranging from messing up the Escort Mission, getting hit by a particular attack, or fighting the enemies in the wrong order.
The Scrappy: Nana Gray and/or XNFE. The former because she sold you to the enemies and for being an all-around AngstyJerk Ass; the latter because she killed all your friends, especially Rothcol.