Less prominent, but Ymir is consequently referred to as Freckled Satan.
"Gesumin" for Armin, when he's being a Manipulative Bastard. ("Gesu" is Japanese for "scumbag".) Coined by Isayama himself, when he posted the original version of the scene where he sports a Slasher Smile while lying to Bertolt about Annie being tortured, showing just how disturbing the original expression was. It was captioned, "This Gesumin....What should I do with it?".
"Geographia" for the woman shown in Eren's flashback in chapter 53, due to her resemblance and possible connection to Historia.
Hey, It's That Voice!: The anime adaptation is almost a reunion of several people who previously worked with director Tetsuro Araki:
Rumored to have stated that any and all pairings may be canon if fans want them to be.
Sleeper Hit: Since its debut, it has quickly become one of the best selling shonen manga in Japan, surpassing other titles like Bleach. See here.
Sure, Why Not?: The 3D maneuvering soldier with afterimage effects in the opening actually wasn't supposed to be Jean, but rather a soldier representing humanity. However, given the number of people claiming it to be Jean, the production staff has acknowledged it as so.
Not exactly from Hajime Isayama, but there was an announce that dakimakura of Shingeki no Kyojin would be released. Of course anyone would expect to see Eren, Mikasa, Armin, Levi, Annie and the like, but when one was released...◊
And Isayama strikes again. He was asked to promote a new series so he drew an advertisement band for the first volume. This◊ was the result◊.
With the release of the second Guidebook, "Outside", the cast now have birthdays. Reiner's birthday is stated to be August 1st, which is celebrated as "Yaoi Day" by fans of the genre.
It gets better: Marco's birthday is revealed to be June 16th. That's exactly half of the year.
What Could Have Been: Hajime Isayama first brought his pilot to Shueisha, with the hopes of having it published in Weekly Shonen Jump, but the editor who evaluated his work said that it was good, but not good enough for Jump standardsnote "This is manga, we want JUMP!" being his exact words; then, when Isayama gave his pilot to Kodansha, it was accepted in no time and published at Bessatsu Shounen Magazine, a monthly offshoot of their Weekly Shonen Magazine. The first two volumes sold more than a million copies combined. As a result, Jump, and consequently Shueisha, lost a giant new hit to its biggest rival.
It's not the first time that Shueisha's heavy-handed editorial policy resulted in losses of the high-profile properties in recent times. Just see the whole kerfuffle with Yukito Kishiro moving Gunnm, previously one of a flagship Shueisha properties, to Kodansha over a dispute about the copyright and editorial choices.
Though lets be fair to jump, the art at the beginning of the manga was really sub-par for a published work, with characters heads being wildly out of proportion, and poses being very stiff. It got better, as art usually does once a mangaka gets in the rhythm of drawing serialization, but even though the story was always intriguing, if you're trying to get published in a big time magazine, presentation is gonna count against you.
The pilot itself, now called Attack on Titan: Volume 0, is extremely different from the series that we know today. For starters, third-dimensional maneuver gear don't exist in the pilot, and humans are fully capable of "jumping 10 meters" while fighting Titans.
Isayama had originally planned to kill off Sasha in chapter 36, but later abandoned that idea.