Fan Nickname: In her episode recaps, Strega of Television Without Pity refers to the good guys as the MoG, which is an abbreviation of "Ministers of Grace" ("Angels and ministers of grace, defend us" - Hamlet).
Strega also (rather unkindly) took to calling Spike "Spicule", apparently a dig at his Badass Decay.
Lorne is nicknamed "Cary" (Cary Okie. Geddit?) for no particular reason other than it sounded better than The Host.
Genius Bonus: Izzy, a Devil in business attire. Throwaway gag right? D&D players delineate between Demons, who are Chaotic Evil in alignment, and Devils who are Lawful Evil. Hence, a devil working at a law firm.
Although many would recognise him more for his voice (as mentioned below) Simon Templeman as Matthias Pavayne would also be recognisable to viewers of Charmed, in which he played the Angel of Death in three episodes. He also appeared as Lord Nor in two episodes of Lois and Clark.
The gym manager in "Judgement" is played by actor/stuntman Keith Campbell, best known for playing the alien perp who leaps off the building in Men in Black.
John Kapelos, the hotel manager in "Are You Now Or Have You Ever Been", formerly played Donald Schanke, Nicholas Knight's partner for two seasons on Forever Knight. Possibly as part of an in-joke, there is polka music playing in the hotel in one scene: Schanke was a big fan of polka music in FK.
Metal Gear fans will recognize Jim Piddock, the creepy butler guy from "The Trial", as Major Zero.
Real-Life Relative: In the scene with Angelus and Faith in a donut shop, the cash register is robbed by Eliza Dushku's big brother, Nate. ("Orpheus")
Retroactive Recognition: Jennifer Garner can be spotted (sort of) amongst the ladies pregnant with demonspawn in "Expecting". You wouldn't know it's her if not for IMDB.
Bobby Bacala plays one of Little Tony's mooks with a gift for the obvious. "It's Herb Saunders."
Software king David Nabit is played by Michael Bolton. (hee)
River Tam played a cursed ballerina in season 3 episode "Waiting in the Wings".
A "blink and you'll miss it" role: The Progressive lady, Flo, shows up as a clerk in Wolfram & Hart's records division.
Another blink and you'll miss it is Renee Walker appearing among the first group of people mesmerized by Jasmine.
What Could Have Been: There were two different plans spoken of by the writers for how to handle the Fred/Illyria situation in Season 6, which were:
1. Willow would visit the show and separate Illyria and Fred.
2. Illyria would begin to absorb Fred's personality.
Production hoped to get Sarah Michelle Gellar to reprise Buffy in a Season 5 guest spot, but she was busy filming The Grudge during the only part of the season it would've fit best.