Precision F-Strike: Has a few throughout the story, but her most notable thus far is in episode 5: "Then I'll tell everyone at school that all you geeks ganged up on her and fucked her!"
Inverted by her Drill Sergeant Nasty impersonation; when she stops using the F word and other strong profanity in her dialogue, you know the SOS Brigade's about to be eliminated from a baseball tournament in the very first round if they don't shape up real soon.
Seven Dirty Words: Almost; while she does call Taniguchi a motherfucker in episode 9, the latter drowns her out. In a straighter example, she has a tendency to say the most functional word in the English language, "shit", from time to time.
Shout-Out: In episode 8, she paraphrases a line spoken by the most famous Magical Girl of the '90s: "Hey, you were looking up Mikuru's skirt! You pervert!" Even funnier, the offending mascot says the exact same thing each time.
She also says "All right, bitches!" right after scoring a double against the Kamigahara Pirates.
Tights Under Shorts: Wears red tights under her cuffed jean shorts (which are themselves a Flanderized carryover from such anime episodes as "Bamboo Leaf Rhapsody" and "Remote Island Syndrome Part 2") when it isn't summer.
Word of Dante: Or at least an amalgamation thereof, his name being an cross between two assumptions of his names (Kiyohiko, from one source, and Nagaru Tanigawa, from another source— which, incidentally, is not by any means Word of God).
Bifauxnen: Not exactly; it's implied that Cure Extraterrestre does get mistaken for a boy by the others due to that flat chest of hers being highlighted, but the narrator knows better. Then she removes her mask...
Foreshadowing: Kyon notices a vague connection between Yuki's voice and Cure Extraterrestre's voice at the start of episode 7 when Yuki tells him to read The Martian Chronicles that night (and it definitely isn't the extreme monotone voice she'd usually use in her civilian form).
Meganekko: Until the altercation with Ryoko Asakura.
Not So Stoic: A slightly obvious example in episode 6 when Haruhi declares it the SOS Brigade's mission to mingle with aliens, time travelers, and ESPers.
Samus Is a Girl: Subverted; while the narrator refers to Cure Extraterrestre using feminine pronouns, the characters only learn her true gender when Ryoko Asakura accidentally lets slip that under that helmet is the face of a girl.
Sophisticated as Hell: Sometimes delivers Totally Radical dialogue in the midst of all her technical and otherwise sophisticated wordings, always while retaining her deadpan. For example, from episode 3:
Yuki: Go ahead. But please, whatever you do in here, let me read. Number one, this is the Literature Clubroom, what did you expect? And number two, I like to read.