The fight between Banagher and Loni, and the fight between Unicorn and Banshee against Neo Zeong were somewhat underwhelming for some fans.
Those expecting a glorious final battle between the Nahel Argama and Rewloola probably didn't expect the latter being blown up by the Colony Laser for simply being in the way.
Awesome Music: Hiroyuki Sawano's score is probably the most intensely cinematic of any Gundam series, OVA, or movie so far, which says quite a bit when you consider that past composers include Yoko Kanno, Shigaeki Saegusa, Toshihiko Sahashi, Ko Otani and Kenji Kawai.
"Unicorn" - the one used during Banagher's very first sortie in Episode 1 - is especially kickass.
Mobile Suit- Played during the Kshatriya's first appearance, counts as well.
Banshee The theme of the mecha of the same name piloted by Marida (and later Riddhe). One of the most ominous pieces in the entire soundtrack, and that's saying something.
Full-Frontal is definitely no slouch in this department, it being the Leitmotif of its namesake character.
"Blind Idiot" Translation: A mild example in the official Spanish subtitles: Banagher address Marida in Spanish as Señora Marida (Mrs. Marida), when the correct way to address her in Spanish should be Señorita Marida (Miss Marida), since, while she's older than Banagher, she's still young and also unmarried. (The Spanish honorific señora is only used for older and married women)
Another annoying flaw in the Spanish subs is the fact the translator seems to translaste the Japanese language's honorific forms almost literally. The most egregious example of this is how Riddhe address Banagher, as he speaks to him as he is a complete unknown to him, making the Spanish subtitles to sound as extremely rude as he already is in Japanese.
Everything surrounding the yet-to-be-revealed contents of Laplace's Box: The original Federation charter that granted more rights to spacenoids as well as confirming the idea of newtypes. However it was stopped by a terrorist attack instigated by more extreme Federation elements who re-wrote the charter to give the Federation more control over the colonies. Some people see this as adding even more to the general Gray and Gray Morality of the Universal Century while others see it as a case of trying to canonize the general Zeon Is Right mentality thereby dismissing it (and the series in general) as "fanwank".
However, the series tries to elaborate on this more by saying that the charter was made completely independent of Zeon's motives, since it was drafted decades before the rise of the movement. Also, Zeon's rise actually made the situation worse, since what started out a simple coverup turned into something that could potentially legitimize Zeon, causing the Federation to clamp down even harder on the Box. Also, when Mineva reveals the original charter to the world, she specifically says she is not seeking to legitimize Zeon's ideals.
To a certain extent, the whole OVA series. Since the OVA is based on the novel instead of original work, a number fans aren't pleased to see some important (and some awesome) contents were cut out, making the OVA series seem like a long Gunpla promotion video, which becomes worse as the OVA goes on. For example, Marida is supposed to have a much larger role during the fight against Full Frontal in Episode 7, has a literal (and better) Heroic Sacrifice, and Riddhe's change of heart is less odd as it's shown. Let's not get into the issue of the superpowered Unicorn in the last Episode; it's significantly more powerful than its novel counterpart.
The recut TV version of the OVAs, Unicorn RE 0096, has also been met with this reaction - some fans are fine with it, pointing out that it means that new scenes from the novel will be animated, some existing scenes will be reanimated, and it's the first UC Gundam TV series in a long timenote Excluding Gundam Reconguista In G, which while connected to the UC timeline, takes place in the Reguild Century; other fans, however, are upset that the series is being recut for TV broadcast, pointing out that Sunrise could just as easily adapt another novel or a manga, such as Gundam Sentinel or Mobile Suit Crossbone Gundam if they wanted to make a new UC Gundam series.
Ensemble Darkhorse: The unnamed Stark Jegan pilot who nearly owns the Kshatriya in the first 6 minutes of the first episode.
Not only did he manage to hold his own against the Kshatriya, but the Stark Jegan's HGUC manual implies that he was one of the guys that attempted to help Amuro push back Axis three years prior. The guy was a true hero.
The Byarlant Custom is actually a tactically inferior suit, with only two guns (which double as beam sabers) and its claws. Its sheer speed and maneuverability (and gigantic claws) make it one of the best combatants in the anime.
The nameless female aide in Cheyenne seen in Episode 7, who seems to convey everything without ever doing anything. So much that fans have dubbed her as Unicorn's equivalent to Kergerenko.
In Episode 7, Bright and Chairman Marcenas find common ground as fathers, and how they shouldn't act in a way that would make their children ashamed of them. This is especially tragic if you consider Hathaway's Flash canon.
If you've seen Mobile Suit Gundam F91 or Mobile Suit Victory Gundam, both of which take place decades after the events of Unicorn, you'll know that the opening of Laplace's Box will ultimately be in vain. But at least there's consolation in that there's possibility again.
This is further compounded considering the situation in Gundam Reconguista In G pretty much suggests Earth and Space goes the way Full Frontal would have wanted: the latter more self-sustaining than Earth ever was.
Even Harsher since...Turn A Gundam is the ultimate end result of Universal Century.
Hilarious in Hindsight: "Full Frontal" might seem odd as the name of a general, but if you've ever seen The Book of Mormon, it's nothing compared to General Butt Fucking Naked. (He, in turn, is based on a very real and very vicious African general, Butt Naked, but that's a little less Hilarious and more Harsh.)
Ho Yay: It's all but stated that Angelo Sauper is in love with Full Frontal.
Riddhe's attitude towards Banagher throughout the series jumps between Foe Yay and this depending on which side of the Heel–Face Revolving Door he's on. Reaches a zenith in Episode 7, especially where his desperation when Banagher is trapped in a Newtype coma verges on I Want My Beloved to Be Happy.
The new mysterious red mobile armor revealed in the trailers for episode 7 has been mocked a little for it's thruster "legs", such as replacing them with soda cans or just generally being confused how the hell it's model is going to keep balanced.
The name, though extremely narmy, likely has some kind of ironic and fairly witty symbolism. Full Frontal implies being "bare naked" or "fully exposed" (*COUGH* ...Which really doesn't help the Narm factor) while in reality the character wears a mask and is very mysterious.
Also, Full Frontal, "full frontal assault,"note ...Not that dammit as in a full-on, all-out assault, so it also doubles as a shout out to the missing-in-action Char Aznable's combat style.
On the other hand, he is aping a guy who spent a couple years going by the name Quattro note Vagina Bajeena.
A rare few moments in the surprisingly competent English dub sounded somewhat phoned in, particularly Marida Cruz's VA's reading of the line "So heavy." being utterly narm-tastic. (Although that might have to do with the line being an awkward one in-and-of itself, being a direct translation of the Japanese audio.)
Episode two looks to continue this tradition further. "That's the See-non-jew."
Narm Charm: Banagher's rather cringe-worthy reaction to Marida's death by point-blank beam shot may seem over-the-top, but really serves to highlight just how powerful a Newtype he is, not to mention how heavy her death weighs on him.
Older Than They Think: One might suggest that Unicorn's verdict on the existence of Newtypes (i.e. they don't exist, or at least are still too human to be considered a new species entirely, in contrast to long-standing Zeonic ideology and Federation paranoia) was pretty much the point made in the climax of After War Gundam X.
True Art Is Angsty: Gundam Unicorn currently has a lot of acclaim, and it's one of the darkest works in the franchise so far.
The Untwist: From the moment the first details about the novel came out, people already knew that Audrey was a teenaged Mineva Zabi, thanks to factors like her appearance, her age matching up with Mineva's, the latter's mysterious disappearance at the end of Zeta Gundam, and the obviousalias.
Another contributing factor is that the score playing when Audrey escapes the Sleeves ship in the first episode is titled "Mineva".
What Do You Mean, It's Not Political?: The novel's portrayal of Zeon and the Federation have been construed as metaphors for Imperial Japan and the Allied Forces, respectively. A more justified instance, considering the author's outspoken rightwing views.
WTH, Casting Agency?: Not casting exactly, but the fact an avowed Japanese Imperialist author like Fukui is writing the novels is rather odd considering the blatant anti-war & anti-Axis themes of previous UC series. For perspective, it's a bit like having a sequel to Maus written by the guy who did The Turner Diaries.
Not a full WTH, but fans were rather confused when it was discovered that Chieko Honda would not be voicing Marida Cruz, despite the fact that 1) she played both Purus in Gundam ZZ, and 2) she was still doing voice acting (even older women in Gundam) when they started production of the OVA... .
The fact that the Puru clone in question is now an adult and thus have a somewhat different voice may have something to do with it.
This was sort of resolved into a case of The Other Darrin now, upon news of her death...
The same thing happened for Mineva/Audrey's VA in Zeta Gundam (Miki Ito), though this is rather because of scheduling conflicts. Still, many fans rather preferred Miki Ito to play Audrey.