Shane Vansen and Cooper Hawkes (Space: Above and Beyond)
It looked like something might happen between them early on, but then nothing. The way it was written though, it seemed like Hawkes was more attracted to her than her with him. Still, I enjoyed the friendship between the two and just wish it developed into something more.
Jack Tripper and Janet Wood (Three's Company)
There seemed to be hints of an attraction between the two during the series, like the computer dating episode, but nothing ever happened. Instead we get those two bores Philip and Vickie, who had zero charisma or charm. I really couldn't see either character falling for them. I blame the producers. They would rather do a crappy spin off then end the series in a satisfying way.
Picard and Beverly (TNG)
Buffy and Xander (BtVS)