Epic 23. At least until the end of the episode, which sets up the Abare Headder.
Broken Base: How Western Sentai fans feel about Goseiger. They either like it and the throwbacks to previous Sentai series (including Zyuranger, Dairanger and Magiranger) or they hate it because of bad characterization, uneven writing, gimmicks and reliance on them. Things cooled down once Gosei Knight joined the cast, but the show is still subject to intense scrutiny.
It doesn't help that it came after the highly acclaimed Samurai Sentai Shinkenger, and when the double whammy of Power Rangers Samurai (the return of the series) and Kaizoku Sentai Gokaiger (pirates, nostalgia and sheer awesomeness) hit in late 2010, they completely overshadowed Goseiger (Although in the case of Samurai, it's because it's commonly seen as a terrible season).
Points generally used in favor of the series include its awesome mecha fights and interesting villains, but offset by criticisms of its weak heroic cast and bad writing.
With the confirmation that the series is to be adapted as the 20th anniversary series, Power Rangers Megaforce, instead of the much more desirable Gokaiger, the criticisms are starting again. Though it was later eased somewhat when it was revealed that Megaforce would combine Goseiger and Gokaiger.
Robogog, the third Big Bad of the series, is a cruel and callous dictator who rules over his empire with an iron fist. Prior to his mechanization, when the people of Matrintis begged him to help them as their city is about to sink into the ocean, he concludes that fear is the most effective way to control everything. Unlike the benevolent Mons Drake and Makuin, Robogog treats his subordinate like crap, such as beating them up, calling them "low-specs", or installing Punishment Bombs on both Metal Alice and Buredoran to keep his power and influence only for himself. When questioned by the Goseigers over his Bad Boss attitude, Robogog replies by telling them about his aforementioned backstory, implying that he has already abandoned what's left of his humanity a long time ago. He even attempts to nuke the entire Japan in an attempt to kill the Goseigers. Robogog's actions makes it very clear that he only cares for no one but himself.
Buredoran, also known as Brajira, is the Sentai equivalent of Lucifer; a cunning and treacherous Fallen Angel with a massive messiah complex who wants to destroy the world and remake it in his image as its ruler. He was manipulating the three villain groups from behind the scenes to gain more power before backstabbing them in cold blood when they were no use of him, including Metal Alice, who treated him fairly while his memories are still in Robogog's possession. After revealing his true colors to the Goseigers, Brajira makes his move by turning Gosei Knight into his lackey, and then have his Dark Headders killed off to become Doomsday Devices for his Evil Plan. He even calls the Gosei Angels "half-assed", mocks the spirits of his bosses for their failures, and dismisses the Goseigers' Armor-Piercing Question as "meaningless". Not even his final defeat at the hands of the Goseigers could stop him, as he pulls a Thanatos Gambit to ensure the destruction of Earth. An Omnicidal Maniac at heart, Buredoran/Brajira is one of the most dangerous Sentai villains of recent times that the franchise has to offer.
Special Effects Failure: Buredoran of the Chupacabra's claws are clearly EVA or a similar material, given how impossibly flexible they are whenever he moves his hands.
Squee!: A blink-and-you'll-miss-it moment in Epic 21. Takeru (not that one, or the other one), the boy who ordered the birthday cake that Eri's rushing to complete, shows up with his baby sister Hiromi, saying that the cake's for her first birthday. The camera pans over, and with uncanny timing Hiromi smiles into it. Cutest. Thing. Ever.note Of course it's probably just the baby's Real Life mommy making faces beside the camera...
They Wasted a Perfectly Good Character: The Landick siblings are the most underutilized characters in the show. One of Goseiger's problems are a little bit too much focus on the Skicks, especially Eri (Gosei Knight as well for being the only character that has a story arc), which is carried over to Megaforce. Agri in particular could play a similar role toBouken Black Masumi as hinted in the first episode but was scrapped later on.
Fans will never know details about Burajira's past aside from him betraying the other Gosei Angels. Is there a catalyst for his traitorous side? Is he a Skick Landick or Seaick? Since the series is over none of this will be answered. On the other hand, some people feel that we didn't really need to know what his deal was.
They Wasted a Perfectly Good Plot: The origins of Knightic Power. It was never really explained. It would be really cool if it was related to a secret or ancient tribe. The same applies to Gosei Knight, who could perfectly be a lost ancient warrior from Gosei World instead of a Sentient Headder.
During their battles against Warstar, Alata had personal battles against Dereputa. The same kind of happened between Agri and Kingugon during Yumajuu arc (but only towards the end of the arc, thus making it another example of this). Why couldn't we have the same during Matrintis arc, but with Hyde? His water based powers and analytical strategic mind would be used in a great manner if put against a stronger Matrintis's general.
The Gosei World is not shown (or at least details on the Angels' daily life or tribe rivalry) at all.
More use of combined elemental attacks would be interesting, like Wind+Ice or Water+Land or Thunder+Water. Those could be used in several creative ways.
Magis, who created a lot of fan speculation, ended up being not very important outside of a few episodes.
Vindicated by History: Goseiger is not that well-received by the fans, especially since it was pitted together withShinkenger and Gokaiger. However once Go-Busters, Kamen Rider Wizard and Megaforce came into the picture it received less hate compared to when it just started, making this trope a common cycle among Toku series in Japan. Getting an (arguably) better translation (by sub group Millionfold Curiosity) may have helped too.
Woolseyism: The Millionfold Curiosity subtitles translate the monster names so that Western viewers can tell what movie they'r derived from.