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.
Awesome Music: Say what you will about the game, but its anime style soundtrack was pretty awesome, particularly for the AOA and their chilling environment track.
Crowning Moment of Funny: While Confusion is annoying, all but incapacitating the afflicted Digimon, it is also absolutely hilarious, replacing the battle menu with nonsensical commands like "Laugh", "Mad" and "Seduct". Choosing one either makes the Digimon attack or, alternatively, yields a completely useless message like "[Digimon name] Shiny white teeth!" or "[Digimon name] Tears are salty!". Even the music goes absolutely bonkers!
Demonic Spiders: The Musyamon in the Gecko Swamp and the Jungle Grave, who hit unusually hard for random encounters for that level.
Purple Numemon, high defense, high attack, if it hits you, your stats are lowered. Oh, did we mention that it is everything you will ecounter at the time where the random ecounter are at best half as powerful as said Numemon. When you're strong enough to kill them in one hit, they become a good source of Level Grinding, giving you 250 EXP.
Amaterasu Server in general, see the example above ? In Amaterasu, everything you ecountered are at least as powerful as said Numemon. There's a reason that the plot ordered you to visit the Jungle Grave right before Amaterasu Server.
Game Breaker: The Counter Crest. It gives the digimon the ability to counter the opponent's attack. What does it make it a gamebreaker? It counters every single attack and deals full damage. It may not be more powerful than Gallantmon's signature counter, but it requires no MP and it's dropped by a digimon that you encounter much earlier than you would even get Gallantmon. It had to be nerfed at the PAL version (It no longer counters every attack, it only has a uncommon chance to happen).
Imperialdramon Paladin Mode is the best digivolution of the game, for a good reason. By the time you get him, your digimon will be atleast level 55.
Cannondramon isn't a slouch either, being capable of hitting almost any 'mons weaknesses, and its finishing move being super-effective against machines.
Nightmare Fuel: Amaterasu Server's version of Divermon's Lake; the reddish liquid where the water would be looks very much like blood. The Divermon happily swimming in it doesn't make things better.
The Jungle Grave. The theme itself is creepy, and the fact that many players probably won't have done the Forced Level Grinding needed to simply survive one second in it means you probably got your butt kicked when you first got here.
The whole premise of getting trapped in a video game is full of Fridge Horror, especially since Digimon Online does not utilize Narnia Time. Just imagine what it would feel like to hear that your friend or familymember has essentially been trapped in a pod, their conscious in a virtual world with no way of getting them out. One random NPC also worries about her being unable to feed her dog now. What if she (or any other pet owner playing the game) lives alone and there is nobody else around to care for her dog...?
The whole Digimon World turning into Oinkmon is quite creepy.
Sidetracked by the Gold Saucer: Many players will be spending more time being a card battle champion rather than, you know, saving the world from an worldwide cyberterrorism that's threatening both Digimon Online world and the Real World itself.