The language the Shadowmen speak is Swahili.
Answering Principal Wood about whether the rise in school violence is normal, Buffy says, "The Hellmouth has begun its semi-annual percolation. Usually, it blows around May." This is an in-joke referring to the fact that season finales, which usually center around Hellmouth problems, are always shown in May, as American [network] shows run from September to May.
The bag Robin gives Buffy had already appeared in Restless (2000). In her dreams, Buffy draws mud from the bag and spreads it over her face.
In Buffy vs. Dracula (2000), Dracula tells Buffy that there is darkness in her power. It is finally revealed in this story what that really means and what that darkness is.
Anya tells Buffy her role on the team is to provide much-needed sarcasm. This is actually true. In season 4, the writers noticed that some scenes involving the main characters could get too mawkish and sentimental, so they decided to bring Anya in to fill the gap left by Cordelia to keep things from getting too hokey. Spike also ended up serving this purpose (although James Marsters defined his role as being the one who had to say "Buffy, you're stupid and we're all gonna die").