Arch-Enemy: With Facebook. Alternatively, 4chan considers Tumblr this. Especially /co/ due to yaoi fangirls and rampant shippers which they feel bring in the more annoying (to them) aspects of fandom and make discussing things besides Shipping a chore if not impossible. The vitriol got even worse once The Legend Of Korra premiered and Tumblr practically exploded in Korra love, fanart and shipping wars.
Within Tumblr, 'Fandoms' (people who blog mainly about their favourite TV shows, movies, etc) and 'Hipsters' (people who use the site mainly for introspection, inspiring images, etc) appear to be locked in a deadly rivalry, which primarily takes the form of fandoms 'hijacking' hipster posts that they feel are excessively pretentious with fandom references, and hipsters... complaining about the fandoms doing this, mainly.
Art Imitates Art: A lot of users post iconic paintings, especially if they're Pre-Raphaelite or Neoclassical.
For the love of god do not make rape jokes, it will only end badly!
It is perfectly OK to blow off some steam and post about how much you vehemently hate something, but jesus tapdancing christ don't tag it. The fans of that thing who track the tag will see it and you will be ripped many new ones.
Changes are never welcomed. Even if it's just a new button.
For all that is good and holy, know the difference between 'you're' and 'your'!
Posting artwork without crediting the original artist ("sourcing"), and by extension, "editing" artwork (ie, turning it into graphics with photoshop). Especially seen as a problem in fandoms of anime and Japanese games, where ignorant (or lazy) users often upload Japanese fanart without mentioning the artist, providing a link to their site, or putting links to the wrong source.
Character Blog: The roleplaying community; to be more specific, there are ask blogs (either drawn, text, or cosplay) and rpers, which are split into independent rpers and members of a specific roleplay group. Probably older than most visitors think, but the majority of roleplayers are characters from newer series.
Sometimes a video is posted but sometimes, alternatively, scenes are made into sets of gifs for quick viewing pleasures.
Most users will post/reblog photos of celebrities they find attractive, sometimes accompanied by indelicate comments or tags about them.
Some users even have a sub-blog as their personal porn stash.
This love for erotic materials is most notable when Yahoo bought Tumblr and people were terrified of rumors that Yahoo was going to get rid of all of the porn. It even prompted the higher-ups at Yahoo to issue a statement assuring people that they weren't going to get rid of the porn.
Dueling Products: Fought with and eventually took over LiveJournal's spot as the fandom location on the internet. There are examples of small to medium size fandoms completely abandoning LiveJournal in favour of Tumblr. One example is iCarly, which had a half-dozen or so active journals with regular discussion turn into ghost-towns, as the fandom quit en masse to join Tumblr instead.
To a lesser extent, deviantART, as many artists abandon deviantART for Tumblr due the tagging system letting your art be seen by more people.
Eagleland: Played for Laughs and parodied, usually with stock photo of a bald eagle juxtaposed in front of an American flag. Example:
Blog Post: Here in [insert country], it is our Freedom Day holiday.
Reply: In America every day is Freedom Day! *Insert Stock Photo*
Emo Teen: A good portion of the Tumblr demographic, these (depending exactly where you are on Tumblr) can sometimes be a minority of users. Others users often insult and quite often parody it.
False Friend: Apparently, Cole Sprouse. We were a sociology experiment. People took things a bit too far, though (ranging from immature edits of wiki pages to death threats).
Fandom Nod: Several shows with big fandoms have their own official Tumblrs.
Fangirl: A majority of female fandom bloggers on the site.
A bunch of fansites following the convention fuckyeah__. Common alternatives are "hellyeah", "heckyeah", or "fyeah", if the person in charge doesn't want to swear in the blog title. Then there's "feckyeahfatherted" "frackyeahbattlestargalactica" "darvityeahartemisfowl" "fokjayolandi" and "fookyeahdistrict9".
and, of course, "flipyeahrichardayoade"
Yaoi Fangirl: There is also a large yaoi fanbase on the site as well. In fact, the majority of the girls on there can and will ship any gay pairing as hard as they can, the more nonsensical the better. There's something of a running gag on the site that, on Tumblr, there is no such thing as heterosexuality.
Food Porn: Many blogs post richly-colored photos of food for seemingly no reason. Some blogs exclusively post pictures of this. People also share recipes, cooking tips and the like, as a lot of food porn posters are "foodies".
Friendly War: On May 2012, Europeans liveblogged about Eurovision 2012. America declared a Banana War on the Eurovision posts by reblogging bananas.
The users often characterise Tumblr as being divided into the hipster side and the fandom side. Which don't like each other very much, generally. The photograph and art tumblrs are generally left alone.
Hollywood Atheist: A number of blogs on Tumblr really, really, really dislike religion.
Hurricane of Puns: The people of Tumblr love puns. It is not uncommon to see a post that has a several-reblogs-long chain of puns about the original subject.
Image Macro: A number of tumblogs are Image Macro factories, of the sort that recycle one image that represents the subject of the meme (generally with a pie-style background in various colours) and put different captions on it.
Know-Nothing Know-It-All: If you ask a social justice warrior what "Check your privilege" means they'll never give a straight answer.
Link Blog: What tumblogs often become in practice.
Mad Libs Catch Phrase: As noted, single topic tumblogs that catalog or reblog a single focused topic, frequetly fannish, are often titled "fuckyeah(subject)" or the more mild "fyeah(subject)", "effyeah(subject)", "hellyeah(subject)", or similar constructions.
Mood Whiplash: Depending on the blogs you follow, your dashboard may have something horribly depressing, some porn, some Nightmare Fuel, and some silly joke posts, all within the space of ten posts. The same thing can happen while doing an Archive Binge of somebody else's Tumblr.
Moral Myopia: Many "social justice" bloggers think nothing of trolling and harassing user who have offended them...but if someone does it to them, well! That's criminal harassment.
NSFW: Dear lord is it not. Best not to do searches while at work or around the family, lest you run into porn (of all shapes). Fortunately, there is an app out there to save you from the nasties or whatever you don't want to see. Except for the things that were not tagged correctly.
The kinds of blogs you follow can help cut down on anything you don't want to see, but it can still go from puppies to porn in two seconds.
There's a lot of otherwise unremarkable blogs with luridly NSFW titles like "crackofsatansass" or "dave-striders-throbbing-cock". (For some reason, there seem to be a disproportionate number of them involving Homestuck characters.)
Poe's Law: The sheer amount of relative extremism over literally anything - fandom wars, political opinions, et cetera - can make it very hard to determine who's actually being serious and who's just seeing what they can get away with.
Political Correctness Gone Mad: Tumblr is home to left-wing, anti-oppression bloggers who are disparagingly known as "social justice warriors", and they are often accused of being this due to their practice of angrily "calling people out" for not recognizing privilege. Many of them migrated from the social justice community on LiveJournal, which used to be as big as the Tumblr SJ community currently is.
Tommypom, Tumblr's more or less official mascot, is a Pomeranian owned by one of the site's engineers. The staff likes to show him off in their own personal blogs, as well as using him to promote the site and notable editorial stories.
Rule Number One: A surprisingly big amount of Tumblr users seem to blindly follow this one.
Scenery Porn: There are quite a few Tumblrs dedicated to pretty pictures of scenery. Sometimes with a theme, such as skyscrapers, scary-looking places, or abandoned buildings. Sometimes not.
As mentioned in the description, shipping is also very serious business at Tumblr.
Soap Box Sadie: While not as big a presence as amateur photographers, it's very hard not to run into social justice activists on Tumblr.
A lot of people who primarily post other stuff — fandom stuff, art, Scenery Porn, whatever — will dabble in social justice blogging (or reblogging), in addition to the more dedicated activists.
There's a divide between radical feminists and the "mainstream" SJ movement (particularly on transgenderism and homosexuality). In fact, there's usually a divide on about any issue - it's not uncommon to see bloggers fight each over about how offensive something is, how extreme one should be when dealing with issues (the aforementioned divide isn't limited to radical feminists versus everyone else), or what direction social justice should take and what issues they should tackle. Also, the plural/otherkin/transabled community and animal rights community try to integrate into the "mainstream" SJ movement — but continue to remain on the fringes.
Suddenly Shouting: When excited, some users tend to start screAMING IN THE MIDDLE OF A SENTENCE.
Take That: From the example chat on the "new chat post" page:
Tourist: Could you give us directions to Olive Garden?
New Yorker: No, but I could give you directions to an actual Italian restaurant
There Are No Girls on the Internet: Averted with this site, where a large majority of the site's fandom-based Tumblrs are run by females (read: yaoi fangirls). Most of the straight porn blogs are actually run by girls.
This can even be seen as an inversion, in fact—it's (usually) not entirely shocking if you have a hard time finding any men running blogs on Tumblr.
It's come to the point where many just assume all users they come across are female unless explicitly stated otherwise.
This, combined with the fangirling over celebrities and huge amount of porn, was rather brilliantly summed up by the statement "Tumblr is like this weird alternate universe where 15 year old girls obsessively stalk 40 year old men in vans".
Oddly, this data has recorded 2% more males on tumblr than females.
Urban Legend: It is trivially easy to attribute a post to the staff blog, or CNN, or any number of reputable sources that never said a word about it. Apparently this is all it takes to convince large numbers of users it's legit.
To figure out if it's legit or not, click the username. If links to the specific post, it's real. If it leads you to the blog's main page, someone's trying to trick you.
It's also fertile ground for more traditional urban legends, usually of the Lurid Tales of Doom variety such as rape gangs using lost children as bait and HIV needles in petrol pumps.
Averted during the Eurovision Song Contest, when you'll be flooded with Europeans mocking the U.S. for not joining the fun.
Also a frequent accusation towards certain social justice warriors who tend to apply the cultural and racial mores of America to the entire website regardless of whether the person they're condemning or not is even American.
What You Are in the Dark: Users can be this, Specifically the ones who run very classy unassuming SFW blogs. The thing is when you like something NSFW it won't appear on your blog (as oppose to re-blogging). HOWEVER the person who blogged that thing you just liked can actually see what you just liked. It can be kinda jarring seeing a born again christian female like that DP/lesbian/gangbang/gorn/ gif set, sleazy story or whatever you just posted.