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.
Adaptation Displacement: Many Western Digimon fans do not know that the series started off as a spear counterpart of the Tamagotchi, or even that the Digimon franchise had a pair of manga, a short film, and at least 3 video games before the premiere of Digimon Adventure.
Archive Panic: 332 episodes, 9 movies, 6 manga series, and 22 video games. If you want to experience everything, good luck.
Base Breaker: Certain digivolution lines that don't fit for certain digimon, while other digimon receive either too much or too little attention, due to being either Palette Swap variants, gender-specific (based on their design or anime character traits) or plain Ninja Pirate Zombie Robot, to the point where the digivolution lines don't make any sense. Some fans even try to exile certain digimon to a specific digivolution line meant for specific human partners (since they usually have one digimon of their own and would often digivolve it all the way to the Mega level).
Canon Sue: Ryo Akiyama, who is a significant figure in two pieces of Digimon canon (the two-part Adventure series and Tamers), and in his home games, he leads whole teams of Digimon composed of all the other main characters' partners in battle, and can keep them all at Mega Level. On top of that, in Tag Tamers and D-1 Tamers, he's actually V-mon's partner pre-Daisuke.
Black Hole Sue: Let's go over that again — in his home series, more specifically in Anode/Cathode Tamer and Brave Tamer, he creates teams of Digimon by collecting the partners of the original chosen children and tamers under his own banner, simultaneously negating the original child's role in the plot. He doesn't get his own true partner until the fourth game, Brave Tamer.
Tsukaimon, a purple Patamon recolor originally intended as an opponent in the early Digimon World games. However, it became popular as the Evil Twin (With An F In Evil) of Patamon. It shows up with surprising frequency in the video games and card games, and is even frequently the "base form" of several other well-known evil Digimon.
Evil Is Sexy - A few humanoid evil digimon can qualify as this.
Fandom Berserk Button: Calling Digimon a "Pokémonripoff" is a very easy way to piss off a fan. Even though both series have similar names and are both about fighting monsters (some of which are partnered to humans), the similarities pretty much end there.
Faux Symbolism: Given how the Digimon franchise (like so many examples of the Mons genre) pulls inspiration from mythologies and religion around the world, it's not surprising that a few Digimon come with built-in religious function or symbolism that really doesn't mesh with the story at hand. Angel and Demon Digimon are especially popular, and the Angemon-Devimon clash at the end of the first arc in Digimon Adventure is a good example.
Just Here for Godzilla: In an amusing inversion, there are several fans who don't really care about the Digimon and just watch for the interaction between the human characters.
To clarify, the first antagonist ever was was the brat Shinichiro in the C-mon one shot, there were organizations of human hackers in Digital Monster Version S who served as the primary antagonists, V-tamer 01 had a handful of hostile tamers, in Digimon World we have Anologman, another hacker, who is also the primary antagonist (and responsible for the creation of Mugendramon, the most powerful digimon).
Similarly not many people realize the term "Tamer" has been used to refer to a child with a digimon long before the release of Digimon Tamers
Pandering to the Base: Every series contains a clear expy for Agumon and Greymon. A hot-headed lead, often sporting goggles and an introspective foil with a canine partner are common, though not present in every incarnation of the series. Said draconic and canine digimon will get all the screentime in the later series, while the other digimon and their tamers will usually serve as token team mates.
They Changed It, Now It Sucks: Many Western fans blindly reject new series simply because they're different in design and aesthetic from Adventure or Tamers.
Toy Ship: Rampant everywhere, thanks to there being about 30+ kids throughout the franchise.
Ugly Cute: Some Digimon pull this off—mostly younger level ones but some older level ones, too.
Unfortunate Character Design: WarGreymon has this in spades. He has a mostly human anatomy. He also eschews pants in favor of a skimpy black banana hammock. The front of said banana hammock features a piece of armor with a very suggestiveshape.
Woolseyism: Some of the changes of Digimon are actually a little more liked amongst the English-speaking community. Part of it is Nostalgia, but even after seeing subtitles, some actually still prefer the English names.
Most commonly, Myotismon - adding in a bit of a Genius Bonus. A good number of fans still prefer Crusadermon to LordKnightmon, and even like that Frontier made the feminine looking pink-armour-clad knight female. Similarly, Duftmon was localized as "Leopardmon", which given its "Leopard mode" actually makes a lot more sense. GuardiAngemon as well, due to sounding more like "Guardian Angel" rather than "Slash angel".