Joss Whedon has confirmed that April the robot was written (along with "the creation of Warren the villain [and] the Buffybot") with Britney Spears in mind as he considered her performances 'robotic'. However she turned it down, according to David Fury, because she wanted to play a character who hung out with the Scooby Gang rather than a 'sex slave robot'.
According to David Fury, when he questioned the incredible technological abilities of Sunnydale residents (reanimating the dead in Some Assembly Required (1997), constructing lifelike robots in Ted (1997)), Joss Whedon replied "You're just way overthinking it. The Hellmouth should be able to provide us with anything we want to do; the energy that comes out of it makes mad scientists out of humans who then go ahead and create something evil".
Dawn mentions that she always thought Ted, from season two's Ted (1997), would come back. This shows that although she was not actually present for any events prior to Buffy vs. Dracula (2000), she nonetheless remembers them.
Buffy says her secret to attracting men is to "Slap them around a bit, torture them and make their lives a living hell". She has actually done this to all her boyfriends/lovers.
Willow is attracted to April, the first time she ever expresses an appreciation of a woman other than Tara. In season 7, she will be attracted to Dawn and her new girlfriend Kennedy. And in Orpheus (2003) she's openly (though jokingly) flirting with Fred.