Some of these never really got to be, so I'm just wild-guessing at what might have gone wrong!
Bangel: Passion/Lacking in substance; a lot of emotion without that "tried and true," tested companionship
Cangel: Its foundation is a wonderful friendship/Never got to determine extent of "romantic" feelings
Cander: Sweet; first "real" requited feelings for both/ wasn't the right timing for them. Maybe if it'd been 5 years later.
Xanya: Real companionship; real love/ again, timing wasn't right.
Dander: Might really be something there; they've certainly been through a lot together/not quite sure I'm ready for this, even though it's definitely happening!
Woz: Adorable; shared interests, sense of humor; tons of potential/ maybe Willow would have met Tara anyway, and fallen in love with her just as she did? Or the werewolf thing would always be an issue?
Tillow: Love and passion/lies, different values re: magic
Killow: Um...comfort?/ what's it based upon, again?
Sprusilla: Served its purpose; kept two evil and lonely un-souls together in a fiery passion for a century/not a spiritual or intellectual match; no real companionship, (which isn't surprising for 2 vampires is it?)
Batsu: A nice, fun experience for both?/Nothing to it, out of thin air... felt like it was done simply to sell more comics, which...ugh.
Fresley: Beautiful, seemed on the verge of something real/ Ilyria.
Frunn: Cute, happy/just not meant to be
Darla/Angel (the vampire years): Um...the time they spent together potentially distracted them and meant slightly less victims?/Fickle, no love, (i.e. worse than Sprusilla in that regard), 2 vampires, etc.
Jiles? Goyce?: Seems like it'd be a great idea; 2 of Buffy's lonely parental figures uniting/ would have made things harder for Buffy in some way?
Jiles? Genny?: Shared interests, shared sense of humor, passion/ could have disagreed on many things, as time went on
Weslah: They needed each other/ Weslah.
Nangel: Who knows?/Who knows?
Spuffy: Real; companionship; tested; transformations for the better/how often will the past make things difficult in the future?