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.
Anti-Climax Boss: Mewtwo in Pokémon Stadium, 1 of him against your 6 (and said 6 can include 1 of him)? Pitifully easy compared to the cheaters you have to beat to reach him. Only using rentals makes things harder, but he's still much easier than the previous foes.
Breather Boss: Jasmine and the Kanto leaders in Pokemon Stadium 2, due to being the only trainers in their gyms. They're not neccesarily easy, but at least you don't have several other trainers to battle again if you lose to them.
Easy Level Trick: Having trouble Jasmine constantly switching? Entry hazards and trapping moves work very well, since her Pokemon cannot learn Rapid Spin.
It's Short, so It Sucks : A common complaint about Battle Revolution is that beyond its Battle Frontier-like area, there is pretty much nothing else to do the way there is in the Stadium and Colosseum games.
In the Stadium games, Nidoking acts like Large Ham. When it enters battle, it roars and laughs to the sky. When it gets knocked out, it dramatically puts its hand on its chest and gives an expression as if to say, "And....scene."
Vileplume has a knock out animation similar to Jynx, in that it mysteriously disappears underneath its flower. However, it becomes pretty funny once Vileplume's petals start thumping against the floor, almost as if it's "tapping out".
Nightmare Fuel: The one-hit KO animations, especially when they connect.
Jynx's fainting animation, in which it cries, suddenly freezes and its entire body disappears; that is except its hair, which simply drops on the ground. Brr.
Misdreavus' fainting animation is similar: it blinks out of existance with a Tear Jerking look on its face, leaving its string of pearls, which falls to the ground and shatters.
When Arceus or Giratina use Shadow Force. EspeciallyGiratina.
Kangaskhan looks like she's choking when she faints.
When Magnamite, Magneton, and Porygon fainted, their entire bodies broke apart. All three were updated in later games to simply fall over with "X" eyes.
Abra's creepy spasms whenever it got knocked out.
The Gym Leader Castle map in the first Pokémon Stadium. You have a path with 8 gym towers in a cavernous area◊ with smoke all around that seems to lead into a volcano that actively has smoke and lava pour out of it. Then when you beat the 8 gyms, the smoke from the volcano reveals the Elite Four Castle built over the volcano.
When you take a look on the Elite Four Castle map, you'll see a ghostly-looking Venusaur on the left of the path to the Elite Four Castle entrance. The creepy cavern/volcano vibe may be why the entire Gym Leader Castle in Pokémon Stadium 2 was made to be less scary in appearance.
Tear Jerker: In Battle Revolution, poisoning will cause him to say "___ is losing its health!" Mind that he says the last three words in a very dramatic way, as if he's about to break into tears.
Some fainting animations can come across as this, especially if they're of your favorite Pokémon.
That One Boss: In both Stadiums, the final trainer (a male Cooltrainer) of the Prime Cup has a Mew with a wide movepool. No matter what, he'll always use it as his first Pokémon in every match. Good luck defeating it, especially if you're using a rental team.