Abridged Arena Array: While you can certainly find servers running other maps, there are quite a lot of 24/7 Island or 24/7 Arena servers out there.
Complacent Gaming Syndrome: Through either personal preference or some idea that ranged weapons are just "better" somehow, a lot of more inexperienced players refuse to play anything but the designated Long-Range Fighter classes. This can lead to entire teams filled with nothing but Archers or Sharpshooters, who then get confused when a well-coordinated Viking team tears them apart.
Fan Nickname: Several. The "Badass Cutlass" for the Captain's primary weapon, due to it's high damage.
The Archer's Shortsword is the "Butterknife", while the Sharpshooter's Dagger is the "Letter Opener"
The Berserker's Sword/Axe combo is also commonly known as the "Lawnmower" or "Meat Grinder"
Besides, Ninjas are already in the game. You just can't see them. If you ever were to see a Ninja, they'd have to be dead already. And if that were the case, they wouldn't be a very good Ninja, now would they?
Game-Breaker: Before version 2.3, the "Perfect Parry" could negate almost all damage from an attack, as well as leaving the opponent stunned and vulnerable to attack. To make matters worse, the two-handed weapons, in addition to dealing more damage, were also better at parrying than one-handed weapons. The Perfect Parry system was eventually toned down.
Memetic Mutation: Several of the placeholder voicelines present in the older versions of the game are subject to this, especially the Vikings:
"You are NO MAN!"
"Odin's Pizza Place!" Explanation "Odin bids us pace!"
"Ragnarök is a pony!" Explanation "Ragnarök is upon you!"
Sequel Displacement: A lot of people who pick up PVKII wonder why they never heard of PVKI. But condidering it's a mod for the original Half-Life and - unlike its sequel - isn't available on Steam, it's somewhat understandable.