Hey, It's That Voice!: Being the second major FUNimation dub, it shares many voices from Dragon Ball Z. Botan is ChiChi with a British accent, Hiei is a significantly more badass version of Emperor Pilaf/Android 17 and Genkai is a more feminine and less androgynous version of Frieza. Kurama also gave out a hardy Tien yell during his final strike against Karisu.
According to one of the commentary tracks, this was a huge problem for the casting committee; the sheer number of characters in DBZ meant that pretty much everyone had already used up their "hero" voices, so they had to dig pretty far down into the well to keep from being a total clone. They finally told the line producer, Justin Cook, to read for Yusuke, as he'd only ever had bit parts on DBZ.
Oh, and remember the guy Kurama fought in the episode where we saw Youko Kurama for the first time? Yeah, it's even harder to take him seriously once you can tell it's obviously Edward Elric. Kurama versus Edward, that'd be something ...
Younger Toguro and Sensui are brothers in the 2003 anime version of Fullmetal Alchemist. No, seriously - Dameon Clarke (Younger Toguro) still plays a younger brother, Scar, and Robert Mc Collum (Sensui) is Scar's unnamed older brother. They don't retain this in Brotherhood because Scar was recast.
And Yanagisawa, the guy that has the copy power in the Chapter Black arc? That's Colonel Mustang. Continuing with the Chapter Black correlations, "Doctor" of Sensui's team is Hughes.
What Could Have Been: On one of the DVD commentaries, three out of the four main voice actors admitted they wanted different parts from what they were given. Justin Cook (Yusuke's VA) wanted to be Kurama, Chuck Huber (Hiei's VA) wanted to be Yusuke, Christopher Sabat (Kuwabara's VA) wanted to be Hiei, while John Burgmeier (Kurama's VA) didn't admit that he wanted any particular part. If this were the actual cast and Burgmeier was still part of the main four, he would have been left with Kuwabara.
Originally Hiei was introduced as a one-off that was popular enough to keep.
Later, some official artwork had Kurama and Hiei be physically close that fans noticed. When asked if Hiei and Kurama were together, Togashi said he hadn't originally intended them to be a couple but could see the possibility and wouldn't have minded. Sensui and Itsuki however, are an Official Couple.
There is a small short comic that Togashi only gave out to a handful of people. It contains genderbent Hiei and Kurama and a sketch of Kurama and Karasu as a couple, among several other author's notes.