There's a house in one of the routes with nothing but a man and many Pikachu inside (the "Pikachu Fan Club"). He claims he wants to challenge the League with a team consisting entirely of different Pikachu. You're probably seeing his entire team at that moment. Then you try talking with every Pikachu just because... It turns out one of them is actually a Poké Kid in disguise, a kid trainer who dresses like a Pikachu. Who immediately challenges you, like usual. And guess what her team consists of?
The Resort Area contains the first (so far one of only two) instance where you can encounter a level 100 wild Pokemon. Under most circumstances, this would prompt an Oh, Crap! from the unsuspecting player who encounters one of these beasts, as a level 100 Pokemon would be hard to run from or defeat without a similarly-levelled Pokemon of their own. That being said, the encounter is actually hilarious if you're lucky enough to get it - because it's a level 100 MAGIKARP!
Regigigas in Platinum is a pretty hilarious Anti-Climax thanks to it's hilariously nerfed level. Normally, in Diamond and Pearl, this legendary is level 70. In Platinum, however the legendary is Level 1, so The big Daddy of Golems starts with the level of an infant Pokemon. Those who expected an awesome, over-powered Legendary (especially for what you have to go through to even fight it) are truly in for a surprise.
One of the Pokémon who has gender differences is Wobbuffet, where the female apparently wears lipstick.