On the subject of Buffy's fifth season I think that there is something of a fifth season curse on American genre series.
Babylon 5 for instance its fifth season was the worst. Apart from the G'Kar/Londo scenes (which are always pure dynamite) its kind of a bit dull and uninspired. The telepath story arc is as legendarily shit as everyone says too.
Xena season 5 is possibly the worst season of any classic genre series (apart from DW S8) My god its just so shit! From how lame they make the gods, to the baby story arc, to Eve and Livia (dreadful character played by an unbelievably inept actress), to the godawful way they wrap up the Xena/Callisto feud (which was as good as Holmes/Moriarty, Batman/Joker in its heyday)
Once Upon A Time's fifth season is also shit. Hades is fucking lame. His whole motivation is just to fuck the Wicked Witch of the West. They also waste Peter Pan and Robbie Kay which I can't forgive LOL, they make Rumple go evil for no reason after his big redemption story arc, and worst of all they do stupid things like have Cora ascend to Heaven, whilst Milah and Dorothy's granny go to hell!
Finally I noticed how shit Charmed got in season 5 on my latest rewatch. They make the sisters into such bitches the way they treat Cole. Why did they think he was evil? He was only evil because the Source possessed him? Then after making him a good guy for the season he just goes mad for no reason. It really was lame.
Buffy season 5 I wouldn't say was anywhere near as bad as these other seasons, but I do think that the cracks that would affect later series began to appear however, like Spike being in love with Buffy, more focus on the soap opera elements, Dawn etc.
I suppose Angel is the only one I can think of that not only avoids this rule, but ironically season 5 is probably the best season of Angel (or Buffy IMO)
Still I think just as there is a third film curse on film series then there is a 5th season curse on many American genre series. They just seem to not only run out of steam in those seasons, but make their craziest and stupid decisions that year.