There are many things that I think Buffy did wrong, but if I had to pick one it would without a doubt the "Magic Drugs" storyline in S6. In one fell swoop it:
1) Destroyed a subtle, complex, multiple-season-long character arc about power, control and abuse, and simplified it to "MAGIC IS ADDICTIVE".
2) Neutered a once great character by giving her a lame storyline and then turning her into a shell of her former self in the next season.
3) Gave way to Dark Willow and therefore some truly horrible dialogue ("Get off superbitch!" Need I say more?).
4) Corrupted a once beautiful and pure relationship in Tara/Willow and killed my love for them.
5) Forced a sympathetic character to become a villian by being incomprehensibly OOC - Amy, once a shy, cute witch whose abuse of magic extended to pushing back homework deadlines, gets turned into a rat, wakes up 3 years later and is suddenly an out-of-control
drug addict.
6) Went completely against all prior and subsequent mythology of the series. Nowhere, outside of that storyline, was magic ever portrayed to be addictive. Nor did it give users acid trips.
7) Deviated from the show's tradition of tongue-in-cheek self awareness by taking itself completely seriously, despite the stupidity of it.
8) Insulted my intelligence, your intelligence... hell it insulted my pet rock's intelligence with its ham-fisted, anvilicious, in-your-face, after-school-special message that MAGIC IS DRUGS AND DRUGS ARE BAD.
1) Destroyed a subtle, complex, multiple-season-long character arc about power, control and abuse, and simplified it to "MAGIC IS ADDICTIVE".
2) Neutered a once great character by giving her a lame storyline and then turning her into a shell of her former self in the next season.
3) Gave way to Dark Willow and therefore some truly horrible dialogue ("Get off superbitch!" Need I say more?).
4) Corrupted a once beautiful and pure relationship in Tara/Willow and killed my love for them.
5) Forced a sympathetic character to become a villian by being incomprehensibly OOC - Amy, once a shy, cute witch whose abuse of magic extended to pushing back homework deadlines, gets turned into a rat, wakes up 3 years later and is suddenly an out-of-control
drug addict.
6) Went completely against all prior and subsequent mythology of the series. Nowhere, outside of that storyline, was magic ever portrayed to be addictive. Nor did it give users acid trips.
7) Deviated from the show's tradition of tongue-in-cheek self awareness by taking itself completely seriously, despite the stupidity of it.
8) Insulted my intelligence, your intelligence... hell it insulted my pet rock's intelligence with its ham-fisted, anvilicious, in-your-face, after-school-special message that MAGIC IS DRUGS AND DRUGS ARE BAD.