Aside from their similar tastes in men…
Well, Drusilla tortured her boyfriend, Angel (s2: BtVS), with holy water and the next day nearly killed him while using a spell used to cure her illness.
Dru later almost killed her and then the world by raising the Judge.
Slept with Angelus on a regular basis while in Sunnydale and killed fellow Slayer Kendra. Is on a lesser level responsible for Spike kidnapping Willow and knocking Xander out, resulting in Cordy getting skewered.
They also got to join forces in Spike-mocking during 'Crush'…
It's a very bright history.
Smitten with Angel during Season 1, Buffy officially started dating him in Season 2. After losing his soul, trying to kill her on numerous occasions and thereby getting sent to hell, they decided to separate during Season 3.
In Season 4 she briefly spent time with Parker before meeting Riley.
Buffy then briefly joined the Initiative and spent a dangerous amount of time in Riley's dorm. They both left the Initiative and continued dating for another year before Riley decided Buffy didn't find him important enough and took off. She died, came back with an interesting suntan from Heaven and is currently with Spike.
Aside from, 'awww, so cute!'? Well, okay. They're both super strong women who've had to put up with Spike and Angel(us). They've dated the same men, even if said men had slightly different personas at the time.
And, most importantly, it's rarely done. And at that, rarely done correctly. (( IE: in character ))