Chris Avellone notes that Gann and Safiya as love interests were originally not constrained by gender, as "that felt true to how [they] perceived love". This was apparently vetoed by Wizards.
Atari insisted that the Mysteries of Westgate DLC ship with a DRM scheme. This caused the package's release to be delayed for nearly two years, causing an uproar in the fanbase. By the time Mysteries was released, people had lost interest in the adventure pack. Months later, Atari removed all manner of DRM from the game and its expansions, including the DLC. Such a shame, because Mysteries of Westgate was meant to be the first of many Adventures Packs, none which saw the day of light.
Fan Nickname: The player character is often called KC by fans, which has the advantage of being an abbreviation for both titles by which characters frequently refer to them in the first campaign (Kalach-Cha and Knight Captain).
The Other Darrin: The voice actors of both Kelghar and Bishop changed between main campaign and expansion. Kelghar's voice actor in Storm of Zehir is noticeably different, with the new guy trying to imitate Adam Sietz and not quite succeeding. Bishop's new actor in Mask of the Betrayer lacks the old one's sneering, aggressive edge.
Screwed by the Network/Screwed by the Lawyers: Hasbro/Wizards of the Coast sued Infogrames/Atari over fraud and five breaches of contract. Continuing game updates afterwards was likely not a high-priority issue. It also resulted in the Steam version being pulled from the store, though it's since been rereleased on GOG.com.
Lisa Emery voices both Zhjaeve and the succubus Blooden. It's possible to have Zhjaeve in the party when you go to Ammon Jerro's Haven, where you can meet Blooden.
In Mask of the Betrayer Dave Walsh voices both the player character (assuming the Male Methodical Caster voice) and Bishop. Nan McNamara does the Female Methodical Caster voice and the telthor wolf Nakata.
Troubled Production: Mysteries of Westgate didn't take long to develop. However, Atari wanted the DLC shipped with a DRM scheme. This delayed the release of the game. Adding to this, Obsidian's patches constantly broke down the Adventure Pack, pushing the game's release even further, much to gamers' frustration.
At Crossroad Keep, there's a sidequest from Elanee to plant a garden that was cut. The PC could've also set up a gambling den, and been given control of their faction's operations on the docks in a section similar to managing Crossroad Keep. The structure that would have been your base is still in the game next to the Sunken Flagon. Quite a bit of dialog in Act 1 and early in Act 2 still references your character as being in charge of a faction there, even though you never have any authority in the final version.
Back in 2009, there was talk of a possible third expansion and another patch for the game, but everything went down the drain when Hasbro (owner of the D&D licence) sued Atari (the game's publisher). By the time the dispute was settled, everybody had moved on to other games.
In Mask of The Betrayer, Word of God spoke of many things that were cut. Kaelyn was suppose to be a romance option, and Gann and Safiya could originally be romanced by the PC regardless of gender. Kazimika was suppose to ambush you after you devoured the Wood Man's spirit. An extended exploration of the Astral Plane was also trimmed down for pacing. Kaelyn's grandfather was suppose to rescue her if you fought her after siding with Kelemvor. You would had a chance to let Kaelyn leave unharmed... or slay his grandfather and the entire Menagerie. For extra evilness, you could have slain her or let swarm of demons grab her and torture her to death.