Author Phobia: JMS has said that Londo's dream of standing on his homeworld and watching the sky slowly fill with the Shadow ships was based on one of his own recurring nightmares. He had been curious if other people would find it as terrifying as he did.
The character of Warran Keffer existed purely because of this, the studio demanded a hot-shot crack pilot character. Naturally, he got killed off at the earliest possible opportunity. Actually he wasn't that bad a character all told, though some of his dialog was rather cheesy.
The reason why the end of season 4 seems so crowded, and the beginning of Season 5 seems mostly composed of filler, is because the show was originally going to be cancelled after four seasons due to the failure of the PTEN network. The events of season 4 (originally intended to end with the episode "Intersections in Real Time") were shortened by four episodes, after which the resolution to the Earth Civil War arc (originally meant for the beginning of season 5) was resolved in three super-dense Wham Episodes followed by the Grand Finale. Then at the last minute the show's fifth season was picked up by cable network TNT — but now there was almost no material left for the fifth season's first half. A new season 4 finale was hastily shot, (the already-shot Grand Finale "Sleeping in Light" delayed to the series' end) and the eleven-episode hole was filled by the telepath-colony arc (originally meant to be just three episodes), the arc of Londo and G'Kar on Centauri Prime (originally cut from season 4), and standalone filler episodes ranging in quality from the yawner "A View from the Gallery" to Neil Gaiman's marvelous "Day Of The Dead".
In the Babylon 5 script books, Word of God states that he had Season Five plotted out... and then his only copy of his notes were stolen while at a con. The resulting attempts to recover them led to the self-admitted awkwardness of Season Five... of course, this still wouldn't be necessary if it weren't for the potential canceling of the season. Executive & Obsessed Fan meddling?
While Londo was his most visible role, Peter Jurasik is a character actor with over thirty years and 70 roles under his belt; for our purposes it's most interesting that he, like Boxleitner and David Warner, was in TRON (as CROM, an accounting program destined for the games grid).
Byron the telepath is also the Medic from Team Fortress 2, among many others.
When the computer is rebooted after a systems shutdown, it comes back online with an obnoxious, stubborn personality voiced by Harlan Ellison.
Ellison also provided the voice for Zooty's talking prop when comedians Rebo and Zooty visited the station.
Ardwright Chamberlain — Nicolai of Shadow Hearts: Covenant — is Kosh.
Mel Winkler, Aku Aku from the Crash Bandicoot series, appears in the episode "And The Rock Cried Out, No Hiding Place".
In the 2002 TV movie Legend Of The Rangers, Sarah Cantrell is played by Myriam Sirois — better known to a generation of North American anime fans as the voice of Akane Tendo from the English dub of Ranma 1/2.
Kathryn Cressida who plays Kat the Bartender, seen in the first two seasons in the Earth Force bar, shouldn't be allowed near the fusion reactor's button or she might say, "Oooh. What does this button do?"
Line To God: JMS was a frequent poster on the Usenet group rec.arts.tv.scifi.babylon5.moderated during the show's run.
Bryan Cranston appears in "The Long Night" as a dedicated Ranger captain.
Throw It In: The Centauri hairstyle resulted from an attempted practical joke by Peter Jurrasik about his idea for their hair, which JMS went along with because he wanted the actors to feel like he valued their input, and by the time the situation became clear it was too late to go back. It did end up working pretty well by making the Centauri visibly different from humans.
Death of Personality: A form of capital punishment practiced by Earth Alliance, actual execution having been outlawed except in mutiny and treason cases. Also known as mindwipe, and first spoken of in "The Quality of Mercy".
True Companions: Most of the B5 core cast is still tight-knit to this day, particularly Claudia Christian, Bruce Boxleitner, and Mira Furlan. Furlan even says that Boxleitner is still their Captain.
Word of Gay: Well, Word Of Bi. For both Lochley (according to Neil Gaiman) and Cartagia (according to Wertham Krimmer). Although in both cases there was definite innuendo on-screen.
Hugo Award: The Hugo award the show won, for either "The Coming of Shadows" (1996) or "Severed Dreams" (1997) appeared in on Ivanova's desk in "Sleeping in Light"
Back when nearly all of the World Wide Web could be linked to on one page, it seemed that most web sites had some sort of sci-fi series tribute somewhere. Star Trek: The Next Generation was a particularly popular choice for enshrinement (it was on at the time, see?). B5 has one up on it, though: the NCSA Mosaic browser had a semi-secret page (about:b5) where the developers expressed their love for the series and provided a few links to fan sites in case you wanted to learn more.