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, as well as having the misfortune of running alongside the much beloved Kamen Rider Double and the equally well-liked Kamen Rider OOO, 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. However it somewhat flared back up again when Goseiger footage was still used in the second half of the series.
For Mecha, Manta Headder, simply because he turns the Gosei Great into a pirate themed mech. One can't help but wonder if its popularity inspired the next Sentai.
Epileptic Trees: Since his helmet was never shown in series, what animal was Gosei Green? His color and tribe made many suspect it was a Turtlenote the fact that Dairanger, a series that Goseiger had a few throwbacks to, had a turtle mecha was one of the reasons for this theory. However, thanks to the Ranger Key Set Lost Edition, his animal is now official: a dolphin.
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...
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.
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.
In the same vein as Knightic Power: The show has tribes associated with the Land, Sea, and Sky. Burajira is a fallen Gosei Angel. It would have made perfect sense to reveal that instead of being a Skick, Landick, or Seaick, he was from an exiled fire-based tribe, the Flamick, as an allusion to another fallen angel.
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.
Tough Act to Follow: This came immediately after aforemetioned Shinkenger, which in addition to being beloved by many Japanese Tokusatsu fans, also served as a gateway for many Power Rangers fans due to Disney choosing to air a re-edit of the original Mighty Morphin Power Rangers instead of adapting Shinkengernote Saban eventually adapted Shinkenger into the aforementioned Power Rangers Samurai after buying the series back from Disney, but Shinkenger still became a Gateway series for many, resulting in many new fans going into this series with high expectations.
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.