Tara and Xander. It's incredible how in more than 40 episodes together they had only a single conversation between just the two of them. Before Tara came along Xander was very interested in Willow's love life (and vice versa). I just think it's ridiculous that his only reaction to Willow/Tara is "girl on girl is hot" and "Tara is nice". Tara and Xander have similar sucky families, similar issues with feeling inadequate when not helping enough with the good fight, heck similar taste in women even. 🙂 If Oz and Xander could remain friendly even after the clothes fluke how come Xander and Tara couldn't even bother to talk to each other?
Also, Giles and Wes, Cordelia and Anya (wouldn't that be a fun conversation?), Lorne listening to Tara and Giles's singing. So many missed opportunities.