I haven't RTFT so if any of this has already been mentioned, I apologise.
hardcore fan for 12 years and have watched it approx eleventy billion times along with all the fandom sites back when I still had AOL dial up
Whedon had to cram his plot lines for the final two seasons (7 & what would have been 8) into one utter wank stain of a season because SMG got too big for her boots and refused to do an 8th season - she was the only one who wouldn't sign. Original plan was Caleb as S7 Big Bad (obviously along with the First) and then a 12 episode run for S8 to deal with the First and the uber vamps.
Spike with a soul swoon Just when I thought he couldn't be any hotter, he went insane in the basement. 12 years on I still love him more than anyone else with Dean Winchester a VERY close second
RE the demon: Spike asked the demon to help him give Buffy "what she deserves" and clearly the demon thought that what she deserved was Spike with a soul - also not all demons are bad (eg Lorne, Angel and numerous others)
The Kennedy plot line again, meant to be dragged out over a big time span, as was the introduction of the all the Potentials. Still dislike her. Tara didn't just die, she was murdered, in front of Willow, who was covered in her blood. Who then went and murdered the fucker.
Poor Anya got massively shoved to one side, although ep5 of s7 was a proper heart breaker for me.