Reddit is a very large news aggregator similar to Digg or Fark.com. Its structure is based around "subreddits", separate topic-specific aggregators which can be easily created. Although it isn't a wiki, The Wiki Rule is in full effect here, since subreddits can (and are) made for pretty much anything. And yes, there's a TV Tropes subreddit.Birthplace and center of worship for the Dark Lord Inglip.Reddit is proven to be actually more addictive than TV Tropes. Don't believe us? Here, try it for yourself, but don't say we didn't warn you. And whatever you do, do NOT piss off Reddit!
/r/AdviceAnimals took its name from the "Advice Dog" image macro, which is now virtually non-existent. More of them now contain humans and most don't give any advice — until the "Advice Mallard" image macros were introduced.
Astroturf: Reddit's founders, Alexis Ohanian and Steve Huffman, admitted to submitting links under various usernames when the site first launched to make it look like it there were more people using it than there actually were.
Cut Short: The infamous 'tree fiddy' (a South Park reference) stories posted by thehealeroftri (and others) open with a typically believable, interesting premise, then right before the climax of the story, typically involve one of the main characters asking for $3.50, then turning into a prehistoric animal and swimming away.
Exactly What It Says on the Tin: Novelty accounts, users who always post with a gimmick, generally have usernames that say exactly what they do. For example, Sure_Ill_Draw_That posts a drawing of whatever comment he replies to.
Frequently-Broken Unbreakable Vow: r/SubredditDrama has a rule against voting down comments in linked threads. However, when a thread is linked, the loser in the argument tends to accumulate a suspiciously low voting total. SRS also has a similar policy against participating in linked threads, but is also frequently accused of brigading.
Getting Crap Past the Radar: After r/creepshots was banned, another similar subreddit, r/cshots sprang up with almost the exact same content, but claiming to be pictures shot by and for lesbian women. After that was banned r/candidfashionpolice showed up, again with almost identical surreptitious photos of young women, with commentary in the style of swishy gay men criticizing their taste in clothes, and even the same mods. It's still up as of this writing.
GIFT: The large userbase and loose moderation leads to many, many examples of the GIFT in action.
He-Man Woman Hater: Quite a few women (and men) on Reddit have complained about the site's skewed demographics (see There Are No Girls on the Internet) and outrageous sexism before. r/shitredditsays is the main hub for these complaints (although it's a subreddit that's widely accused of questionable behavior and comments in and of itself).
"Hot girl posting on reddit? Upvoted! Oh wait you're not very attractive? REDDIT IS NOT YOUR FACEBOOK!!"
"I hate COD because all the games are similar, but I love Pokemon."
"9GAG sucks for stealing our original content. Also, upvote this 4chan image!"
Honor Before Reason: The admins have refused to deal with some of the subreddits that promote illegal activity because it would violate their belief in freedom of speech, only deleting subreddits like r/jailbait after media pressure.
The "SFW Porn" group of subreddits, which unanimously refer to "porn" in the Scenery Porn sense, i.e. pretty photos. Gets especially awkward with r/HumanPorn (which does occasionally have artful nudes) and r/AnimalPorn. This often will annoy redditors, who may not appreciate having the word "porn" in their history, no matter its intended use.
Poe's Law: The initial reaction to r/pyongyang is generally confusion over whether it's serious or not. On the one hand, the idea of a bastion of English-language pro-North Korea propaganda on one of the most mainstream American sites is preposterous. On the other hand, they're very dedicated to the charade.
Rank Inflation: r/trees has a number scale to indicate how high the poster is. The problem is that unlike 420chan's number scale, it's subjective (i.e.  is the highest the poster has ever been, as opposed to adhering to certain objective symptoms). The r/trees FAQ acknowledges that most  posts are exaggerations. Some posters have suggested  or similar to actually refer to being that ridiculously high, but they are quickly shot down.
Self-Parody: There are entire Reddit communities based around mocking Reddit, most notably r/circlejerk. Unfortunately, this often escalates into bullying.
Serial Escalation: When one user posts a certain image, some others decide to post a new image trying to either one-up the previous one, or add [Fixed] to the replacement image. By the time a random reader notices, it's the sixth time they've created a new escalation for a theme, and they've become increasingly efficient about it.
Serious Business: Karma points. Dear Lord. "Power users" with extremely high karma tend to attract a Hatedomjust for having high karma.
Stealth Parody: One interpretation of r/shitredditsays is that its members talk about priveleged people, namely Straight Able-bodied Cisgender White Males (SAWCASMs) the way they talk about minorities on Reddit.
Trademark Favorite Food: Bacon, Nutella. In a more condescending stereotype, Doritos/Cheetos and Mountain Dew, usually to mock game journalists, after an infamous interview from Gamespot where an absolutely miserable-looking journalist conducted a piece with Mountain Dew and Doritos prominently placed in the vid.
Troll: Many subreddits are full of these. It's usually best to assume that "I Am A" and "Ask Me Anything" posts are fake until proven otherwise; as you can imagine the idea is such a lure for trolls.
Trust Password: IamA and AMA posters are encouraged to verify their authenticity by submitting photos with a timestamp.
TV Tropes Will Ruin Your Life: It is considered slightly bad form to post a link to TV Tropes in a comment thread. Not that anyone there doesn't like us; just that once they get here they'll be stuck for the rest of the day.
Most of 4chan deeply despises Reddit, to the point where they've made hundreds of fake versions of Reddit's rage comics to troll themselves with. However, the hatred is generally one-sided. Something Awful hates Reddit for the same reason, on top of all the misogynistic, anti-theist, pedophillic, and creepy (not in the 'horror genre' sense) subreddits. They organized a giant raid to force Reddit's admins to shut down all the child porn subreddits.
Reddit has one of these with 9gag, Reddit having an entire sub-reddit devoted to complaining about 9gag, while most members of 9gag aren't even aware of that Reddit dislikes them.
Reddit has had a long-standing feud with Gawker, which boiled over in the violentacrez debacle.