Now, the reason why this causes me trouble? My favorite character is Buffy. If Buffy is in your fic, I'll want to read it. If Buffy and Spike are in your fic, I'll probably have already read it (even before you've posted it!). *grins*
But I find there's a huge problem for me when it comes to Buffy's portrayal in fic. Buffy is the Slayer. Yet often times Buffy isn't written as the Slayer and that is frustrating for me because her being the Slayer is what brings out so many awesome heroic moments for her. It's why I don't really do AUs - Buffy without her Slayer half, well I love her for her duality of perky cheerleader meets badass warrior. That, to me, is Buffy.
I like my Buffy when she's heroic. Buffy is the Slayer and with that, she always carries this air of responsibility that tempers her fun frivolity.
So if someone's in danger and Buffy hears about it? She's gonna try to save them.
So if there are demons to be slayed (and there always are)? She's gonna hunt them down and then summarily put them down.
If there's a problem, Buffy isn't going to look to someone else to fix it (unless she's suffering from "I just lost heaven" depression). She might enlist others in helping her, but she's still going to be leading the charge. And I've read stories where Buffy suffers through her circumstances and it immediately throws me out. Buffy is proactive! That is the core of her. She's a do-er, not a thinker. When something happens to her that she doesn't like, she tries to change it, she doesn't fall into a pit of neverending mope-age. Carpe diem, Buffy. Remember when her mom was sick? She wanted to solve that, make it about a demon, so that she could do something to help. She's a problem solver, she "help[s] people" and "know[s] how to do stuff." (Anne 3x01)
(This is part of why I'm so glad for the Season 8 retcon of The Girl in Question - that AtS episode depicted Buffy as just a frivolous party girl. It only showed the frivolous side of Buffy, no Slayer in sight. Not exactly designed to please me.)
Often times Spuffy fic feels the need to remove the Slayer aspect from Buffy in order to bring her and Spike together. She'll be injured and need Spike's help or he'll become her caretaker. She'll lose her powers and need him to protect her. Or there won't even be in-story phlebotinum - she just will be written without her inner strength and Slayer core. Now in canon, Buffy might run from her problems (oh boy, she does that a lot!) but in the end she always turns around to face them head on. She isn't a quitter. Or as Spike would say:
In summation, my Buffy is proactive, kickass and a hero. Can haz fic like that, please?