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.
Mark Simmons is undoubtedly the most knowledgeable member of the English-speaking Gundam fandom, and has even done some work for the franchise in helping iron out official translations. He regularly takes part in the fandom, especially in technical discussions, and his website is probably the single greatest source for Gundam info in English, including the official English Gundam site.
He's even gotten some nods within the franchise: Mark Simmons is the default name for one of the Create-A-Characters in the video game Encounters in Space, Gundam SEED has a minor character (a mobile suit engineer) named Erica Simmons, and G-Saviour's main character, Mark Curran, has a sidekick named Simmons.
Broken Base: The fandom is generally grouped roughly based on which series brought you into the fandom, though this has died way down since Gundam's heyday in the early 2000s. The original split was between UC purists and Wingers, with a smaller faction of Seedlings arising later. Gundam 00 was divisive due to its much more Super Robot-styled Gundams and the First Contact plot of the movie given that Absent Aliens has long been a hallmark of the franchise. The newest series, Gundam AGE, has caused some disagreements over its "kiddie" styling (to say nothing of the writing gaffes it suffered in its last two arcs).
Deader Than Disco: Gundam SEED's success would spawn a sequel and a manga spinoff series, and even started talks that the CE timeline could become the new UC. However, Destiny didn't work out so well, and a planned movie to wrap up the saga is in Development Hell, making creators and fans give up on the saga and move on to Gundam 00. Today, if you ask Gundam fans of their opinion of Seed, you will get plenty of contradicting responses, and you certainly won’t find many fans of Destiny.
Fanfic Fuel: Even the original Mobile Suit Gundam made it clear that what was shown on screen was simply one small part of a large war. The UC timeline spans over two centuries, and then there are the Alternate Universe series, and that's not even considering fans making their own Gundam AU. There's also less of an aversion to fans making up all-new casts due to the practice of introducing a new lead character for every anime, manga and video game.
Fanon: The fandom is quite good at creating it, given how many Alternate Continuity versions and contradictory source materials there are.
A similar phenomenon is going on in Gundam Age. Fans who are rooting for Vagan,formerly known as the UE, tend to call out the Earth Federal Forces as corrupt and genocidal. At the same time, they tend to stay mum on the fact that the "oppressed martians" kill millions of innocent civilians, many of whom lived in colonies that were never protected by the Earth Federal Forces to begin with.
Also, some fans will rage against one continuity for having newtype ripoffs, or Char Clone, overly simplistic messages and what have you, and yet will turn a blind eye to it when such themes are re-used ad-verbatim in the original Universal Century timeline.
Misaimed Fandom: Yes, the Federation has corrupt leaders. Yes, Zeon has many good, moral, people who serve it. Yes, the Titans are bad. That being said, it's a little scary how many fans will knee-jerk a SIEG ZEON for a regime that murdered, not millions, but BILLIONS of people and continued to wipe out metropolitan centers every few years... FOR FREEDOM.
Rooting for the Empire: For UC, Zeon has far more vocal fans than the Earth Federation. In CE, just mentioning it in reference to the Earth Alliance or ZAFT is likely to cause a Flame War over who was really the bad guy. Ditto, though to a lesser extent, for Mobile Suit Gundam 00. The same debate over whether Vagan or the Earth Federal Forces is worse in Gundam Age is getting ugly as well.
Tough Act to Follow: Many fans view Tomino's work as this, with anything set in an Alternate Universe very rarely holding up to Tomino's standard. Shows set in Tomino's Universal Century have fared much better, with all of the OV As (none of which had any input or involvement from Tomino) being highly rated by both fans and Tomino himself.
True Art Is Angsty: Zeta Gundam, while having one of the bleakest endings in all Gundam, is generally considered to be one of the best of them all. Gundam 0080 is similarly dark, and similarly exalted. On the other hand, ∀ Gundam is also considered to be among the best, and it's arguably the happiest Gundam series. Ditto for Crossbone Gundam another series that avoids the Kill 'em All status Tomino is infamous for. Incidentally, Tomino wrote all three series. Gundam Build Fighters, which is orders of magnitude more lighthearted than anything else in the franchise, also got a very positive response.