Whistler was originally going to be Angel's sidekick for the Angel (1999) spin-off, however, Max Perlich could not commit.
Whistler jokingly says Buffy was "prettier than the last one"; the Master said the same thing about her when first meeting in Nightmares (1997).
When Giles reveals he has an Orb of Thesulah, he mentions that he's been using it as a paperweight. This is a reference to Passion (1998), in which the magic shop owner tells Jenny that he usually sold the relics to tourists as New-Age paperweights.
Buffy and Xander's fight about whether or not Angelus should be killed for his crimes is much like the one they'll have about Anya in Season 7's Selfless (2002), only the roles are reversed.
Buffy is revealed to have shared many personality traits with Cordelia before she became the Slayer. In Helpless (1999), Buffy remarks that one of the only things that make her grateful about becoming the Slayer is that it made her a better person. Cordelia will feel similarly about herself after coming into the Visions.