Ambiguous Gender: Gender is not displayed on the public profile, therefore, this applies to everyone that does not have a gender-specific username. This provides much amusement for some people.
Ban on Politics: And other sensitive topics such as religion — but these rules only came after too many flame wars began. Not that this stops anyone. Topics that break these rules are fairly common; many of the more lenient moderators tend to avert their eyes until the topics are reported by users.
All links are banned (apart from HarperCollins-owned sites), because anything not created by them could have inappropriate content. They don't even let you say ".com" on the end of a site name. And words such as "youtube" and "proboards" are censored in order to further discourage linking.
And now any mention of another site will be edited out. Threads specifically about another site are locked and/or deleted.
The censor. Censored-out words include "suck", "fatty", or even words like "premature", "headlights", and "lotion", of all things. Amusingly, upon last check, the words "badass", "bastard", "incest", and "douche" aren't censored. Huh.
Subverted by the members themselves. They actually had to create rules saying that there is to be no "forcemating" (rape) in RPs, and that all romance RPs must be PG-rated. Yes, these rules were necessary on a forum consisting of mostly under-13 members.
Any references to homosexuality end with the topic they were in getting deleted. This, despite the fact that there are actually gay members.
Development Hell: Apparently where the online Warriors game is. Of course, due to the fact that fans have been waiting for it since about two years ago when it was announced, everyone's building it up to be a bigger thing than it probably will be.
Digital Avatar: Avatars must be chosen from a preset gallery; you cannot use your own. The only "different" avatars are those from the winners of the Fanatic Challenge (they are the regular ones with a star or paw in the corner).
Double Entendre: Half of the words censored by the forum. Ignoring the fact that the majority of the people within the site's age group would not even know that the words have a naughty meaning if they hadn't been censored.
The Gods Themselves — The Erins (the ErinHunter account).
The Administration — JanaM (formerly).
Moderators — Many over time. Most of them have come and gone, but here's all the ones that they've had in the forum's history: Warriorcats Moderator (aka Dave), Joe S, lisab, Carolyn, Tina, Elizabeth, Ohnsoon , Lori, Mike, Mary Kate, Will, Matthew, Mary, TJ, Alana, Alex.
Guest Posters — Nonexistent. You must have an account (with parental permission if under 13) to post.
Hit And Run Poster: Kit stealers (at least the original few), kit huggers, kit haunters — these all typically are nothing but someone coming into an RP and posting "so-and-so hugs/haunts/etc Examplekit" once.
Internet Tough Guy: Crop up on rare occasions, and usually get suspended/banned for their threats.
Lucky Charms Title / Hešvy MŽtal ‹mlaut: Seeing as there's over 65,000 members, and pretty much all of the names new members want are taken, there are many, many usernames like this. This also appears frequently in "headers" (text people copy-and-paste into the start and end of the actual post because apparently signatures aren't enough).
Mistakenly Banned: Claimed once in a while, though most of the time it's a person who knows why they were banned, pretending to be a friend/relative of the banned account. One amusing real case of someone being mistakenly banned was when they were inactive for a month, and upon trying to log in again, they got the following message: "Account Suspended: Your account has been suspended for the following reason: Hello! This account is being suspended because the owner is reported as deceased."
Moral Guardians: Due to their... curious choice of words that need to be censored, and overall strictness, the mods sometimes come off as this.
Ninja Editing: Done occasionally. Infamously used in the first Fanatic Challenge, when people would make a First Post to guarantee their spot (the point of the Challenge was to be one of the first ten with the correct answer), and then after seeing others post the answer, they would go back and edit their first post, making it appear as though they had the correct answer all along. Resulted in ALL posts, not just ones made after the first minute, having a "Last Edited by xxxx at xx:00" message.
People's reactions to Joe S saying "Stop fighting or heads will roll" would qualify.
The time that the official Erin Hunter account got hacked and started posting crazy "spoilers" for The Fourth Apprentice - for example, that Firestar gets killed by beavers. Many didn't question this information because it was, after all, the real account.
Number of the Beast: In addition to many other odd things they censor, apparently they decided that the number 666 is inappropriate for the site, because they actually censored the number. It appears as *** instead.
Serious Business: People are posting that they stole imaginary roleplay kits? The horror!
Symbol Swearing: The censor replaces swear words with asterisks. Seeing as they censor words that aren't so bad, as listed above, this can sometimes make sentences appear to have a worse meaning than they actually do. (Example: someone's talking about the character Swallowtail and says "Swallowtail is a fatty". This becomes "Swallowtail is a ***" and we'd naturally assume they said a different five-letter-word.)
There Are No Girls on the Internet: Averted — there are many more girls than guys. Inverted, possibly — IIRC, the default assumption is that a poster who hasn't specified their gender is female.
Xtreme Kool Letterz: Since there are many preteens who don't particularly care about spelling/grammar, this comes up pretty often.