Beam Me Up, Scotty!: For fans who read fan fiction, it's an accepted phenomenon that Kaoru's favorite scent is Jasmine (and to a lesser extent, Kenshin is often associated with sandalwood, or something), even though the only character in the ENTIRE series to have a scent associated with them was Tomoe and her plum blossoms.
Creator Backlash: Watsuki has expressed regret at creating Raijuta. He's often cited as saying that Raijuta is his least favorite character.
Yahiko, like Kenshin. In the dubs, it's case by case.
Soujirou in the original.
Executive Meddling: In his original conception, Kenshin was supposed to be ''at least'' 30. But Nobuhiro Watsuki was told that "30 was too old for a primary protagonist in Shonen Jump." So, Watsuki dropped Kenshin's age to 28. (Thus keeping the concept of a more seasoned protagonist than the typical of the Shounen genre, but also making Kenshin young enough to be "acceptable".) Lampshaded in series, when Kaoru tells Kenshin he "can't possibly be 28", and Kenshin asks "Would 30 make you happier?" and Kaoru replies that it would not.
Hey, It's That Guy!: The live-action film involved Kamen Rider Den-O as Kenshin. Bonus in that Takeru Sato, the lead, recently portrayed another historical assassin, Okada Izo, in the historical drama Ryomaden (which was apparently why he was chosen in the first place). The director of the film, Keishi Ootomo, was himself the director of Ryomaden.
Furthermore, a majority of the main cast were alumni of the annual NHK Taiga Dramas (historical dramas of various periods of Japanese history), which include:
Emi Takei (Tokiwa Gozen from Taira no Kiyomori) as Kaoru
Munetaka Aoki (Tosa administrator Goto Shojiro of Ryomaden and Benkei from Taira no Kiyomori) as Sanosuke
Kagawa Teruyuki (Tosa ronin and future Mitsubishi founder Iwasaki Yataro of Ryomaden, as well as Rokuheita from Komyo ga Tsuji) as Kanryu
Image model Yu Aoi (Nagasaki geisha Omoto of Ryomaden) as Megumi
Eguchi Yosuke (Sakamoto Ryoma from Shinsengumi!) as Saito
The aforementioned Koji Kikkawa also counts, considering he played Oda Nobunaga in Tenchijin and now Saigo Takamori in Yae no Sakura.
Retroactively counting for appearing in Yae no Sakura are Go Ayano (Gein, playing daimyo Matsudaira Katamori of Aizu) and Eiji Okuda (General Yamagata, playing pro-Western scholar Sakuma Shozan).
The casting announcements for the upcoming sequels have added more to this, such as:
First and foremost: Tatsuya Fujiwara (Light Yagami and Okita Soji from NHK's Shinsengumi!) as Shishio
Ryunosuke Kamiki, known for voiceovers in Spirited Away, Howl's Moving Castle and Summer Wars, as well as Minamoto no Yoshitsune (both as a child and as an adult in Yoshitsune and Taira no Kiyomori, respectively) shall be Sojiro.
Yukiyoshi Ozawa (Kiso/Minamoto no Yoshinaka from NHK's Yoshitsune and Saigo Takamori from Atsuhime) shall be Ito Hirobumi, the first Japanese prime minister.
Min Tanaka (Tosa councilor Yoshida Toyo from Ryomaden) as Okina.
Hilarious in Hindsight: Watsuki-san noted in his character profiles that he based Kurogasa/Udo Jin-e on the historical assassin Okada Izo. Fast forward today where (as detailed above) Takeru Sato, who played Izo in Ryomaden, plays Kenshin. This comes full circle when the film incarnation of Jin-e, who was based originally on Izo, claims to be a more genuine assassin than Kenshin-as-Battousai. Trippy? You bet.
Back in Kamen Rider Den-O, Takeru Sato played Ryotaro, who is the nominal Kamen Rider, a living anomaly referred to as a Singularity Point. How does one mark a point? With an X.
He was known for saying "henshin", now he goes by Kenshin. Not hilarious but definitely silly.
The author originally intended for Usui to be a Terminator-style manhunter, pursuing Kenshin relentlessly through darkened city streets. Instead, we got theabove.
Henya was originally going to be introduced as a villainous character during the Kanryu arc; also, he was originally going to be in an aerial battle with Kenshin, but Watsuki changed this because he felt Kenshin would have been too strong an opponent.
According to Watsuki, Raijuta was originally supposed to be as imposing as he looked, but Villain Decay gradually set in.
The manga was originally going to be much shorter than it turned out to be, revolving around the Jinchuu arc.
It was originally going to be revealed that Hannya had been stepped on in the womb like the Elephant Man, explaining his featureless death-mask of a face. Watsuki realized the Unfortunate Implications of that idea, and altered the backstory to Hannya having intentionally mutilated his face so that he could easily disguise himself as just about anything.