"They're a wonderful, friendly people—as long as their bellies are full and their holosuites are working. But take away their creature comforts...deprive them of food, sleep, sonic showers...put their lives in jeopardy over an extended period of time...and those same friendly, intelligent, wonderful people will become as nasty and violent as the most bloodthirsty Klingon."
Planet of Hats: Averted; we're the only planet that doesn't have a hat.
Several characters have commented on how relatively fast humanity expanded compared to other species and how quickly humans tend to pick up a skill or job. Humanity's hat seems to be taking everything Up to Eleven.
We All Live in America: Or at least European; all other cultures seem to have died out. All the names are surname-last, alien cultures that borrow elements from other Earth cultures are seen as mind-blowing, and the French all sound like grumpy Englishmen.
Vulcans
"Live long and prosper."
The Atoner: They were once a Proud Warrior Race that was probably even fiercer then Klingons or humans. Horror at the results of this made them turn to the teachings of Surak and follow the rather painful creed of the time of the show to control their violent emotions.
And Romulans are Vulcans who did not follow Surak. Nuff said.
Actually, they are usually portrayed as Worthy Opponents and Noble Demons in both, especially in the latter when in at least one episode Archer gets a Klingon lawyer who serves as a Deconstruction of this (helpfully played by Deep Space Nine's General Martok, Worf's buddy), saying that neither of his parents were warriors and lamenting the way his people are becoming less civilised, as well as less honourable than they claim, but denying that every Klingon is a savage barbarian.
Rated M for Manly: Their idea of a bachelor party is four days of Macho Masochism. Their idea of a honeymoon is going on a hike through the nastiest terrain in the galaxy. Their idea of a joyous wedding night is for the happy couple to gleefully beat each other to a pulp. And their idea of a wedding ceremony is to tell how two mythical Klingons showed their love for each other by teaming up to sack and destroy the heavens. Isn't that romantic?
"We are creatures of duty, Captain. I have lived my life by it. Just one more duty."
Arch-Enemy: On TNG. Not as powerful as the Borg, not as ruthless as the Cardassians, but more recurring than either and are behind half the evil schemes in that series. Arguably this again in Enterprise.
Alternate Character Interpretation: In the novels, they often come across as dignified and traditionalist aristocrats rather then simply as bad guys, though the Romulan characters that appear on TV sometimes do have that aspect to them.
Man Behind the Man: If some villain is implied to have a secret benefactor, the benefactor will probably be the Romulans. Especially if the villain is a Vulcan or a Klingon, just to show how traitorous or gullble they are as both species regard the Romulans as long-standing enemies.
Strawman Emotional: Disagreed with Surak's logic and left to start their own, more amoral, militaristic society.
Though overall, they are still very composed and disciplined. Ironically despite their imperialistic empire, they seem to contradict the idea that Vulcans who don't control their emotions are a dangerous menace, since on a personal level they rarely if ever violently lose their temper or hint at uncontrollable emotions. In-universe this is attributed to their lack of suppression; there is no emotional build-up to blow off when they lose their cool.
Worthy Opponent: Several of the most memorable Romulan characters in the original series, as well as a number of times in the novelizations.
Cardassians
"They were cunning warriors...always had a plan within a plan leading to a trap...it was an honor to kill them."
Mad Libs Catchphrase: The Rules of Acquisition, of which there are over a hundred. Possibly the only code of honor the Ferengi follow.
Subverted by the Rules themselves turning out to be another scam — at least in Quark's dream.
"Would you buy a book called Suggestions of Acquisition?!"
Meaningful Name: The name "Ferengi" is a corruption of the word "farhang," which was a derisive word used in some parts of Southeast Asia to describe European colonialists merchants.
Variations are used across Asia, ultimately all distortions of the word Frank, i.e. somebody from France, mistakenly interpreted to mean all Europeans.
Space Pirate: Their original characterization when they were planed to be serious villains. Implicitly Retconned to be just a few who couldn't make it in "legitimate" business.
Villainous Valour: They take pride in their discipline and prowess and are generally treated tragically rather then as faceless mooks. If they were more chivalrous they would be considered Worthy Opponent s. As it is, even Klingons fear them.
