Five of the survivors have a level where they are guaranteed to appear, and if not controlled by a player, they will die during the mission. Two of the remaining three (Kevin and Jim) share a Downer Ending in the first game, leading players to conclude that Mark is the only true survivor of the games (not that it matters, since none of the characters are referenced in the main series).
A couple of David's SP Items in the first Outbreak led to the now-taken-as-gospel fanon that he's an American Indian.
For some reason, many fans seem to have independently concluded that Linda's last name is Washington. She's never given one in-game.
The unlockable "bonus characters" Mr. and Mrs. [color] all come with a passive damage multiplier that makes them far more effective than anyone else in the game. When Outbreak was being played competitively online with players working to achieve the highest scores and quickest speed runs, using the Mr. and Mrs. skins was considered unsporting.
For whatever reason, David King has twice as many unique abilities as anyone else. He's got a special knife combo that knocks down zombies on the last hit, comes with a folding knife that doesn't take up an inventory slot and which can't be broken or given away, can throw wrenches to inflict a startling amount of damage and stumble normal enemies, can create new weapons like stun rods or improvised flamethrowers, and can repair broken handguns or shotguns. Having a good David player on your team was borderline required for many scenarios.
George and Cindy can team up in File #2 to tear through a lot of opponents. Cindy collects herbs and trades them to George; George turns the herbs into anti-viral pills and loads them into his ampoule shooter. A shot from an anti-virus pill inflicts 1,000 damage, which is roughly comparable to the grenade launcher, and the shooter has a much higher rate of fire.
If you manage to get 5 "Heads" in a row with Jim's coin, you become the most powerful character in the game. This, combined with his fast firing rate, his extra coin on File #2, his dodge move and his melee weapon swing move, makes you an unstoppable force that is able to stun every enemy on the game using even something as weak as a folding knife.
Most Annoying Sound: The AI characters love spouting out their ad-libs. A lot. File #2 turns off the voices in the North American and European versions, making for an eerily silent experience by comparison; unfortunately this also means you can't tell right away if someone's poisoned unless you pay attention to the subtitles. Also, if you ever managed to play online using one of the voiced ad-libs versions, there were players who liked to spam "Help!" even faster than the AI characters do if they felt like it.
The Scrappy: The sniper in "End of the Road", a mercenary who spent his time sniping zombies. We first see him shooting an already limping Linda in the leg. He did take a moment to figure out if she is human, but decided that he'll shoot anything that moves for fun. His only redeeming quality is getting toe to toe with his benefactor for withholding information about the nuclear bombing of Raccoon city, before disappearing altogether. You don't even see him get slaughtered by Nyx in the end.
That One Boss: In "Wild Things," it is possible to imprison the zombie elephant inside his pen. He'll break out eventually, but if you manage to escape the zoo before he does, you'll fight a zombie lion instead. On higher difficulties, that lion is one of the hardest bosses in the entire series; he's fast, can pounce on players, often jumps to the roof of the nearby train station where he can't be touched, is a complete bullet sponge, is very difficult to engage in melee, and is accompanied by up to two female zombie lions which are at least as dangerous as he is. There are a couple of tricks in the final area that you can use to knock a lot of health off the elephant, but they're completely useless against the lion.
The Untwist: Was anyone anywhere confused about who the Axeman was before his big reveal?
Vindicated by History: Not only did the game tanked, but most fans ranked it mediocre or just downright bad. It was to be expected considering most of the game's fun relied on the online mode and it was released in an era which nobody even knew how to connect it, not to mention the AI stupidity in the single player mode. However, now it's not hard to meet someone in the fandom wishing the game could be remastered and released again in HD.