1. Willow turning gay was great from an outside-universe perspective, but kind of silly from a character perspective.
2. Adam is an underrated villain and season 4 is an underrated season.
3. Boo for killing off Tara
4. Spike took over too much in later seasons
1. Totally agreed. Some fans insist that she's bi but from Season 5 onwards, the show took great pains to paint her has purely gay, to the point where she couldn't even consider a relationship with that guy with the magic letter jacket in "Him" without coming up with a spell to turn him into a woman first. Now, the writers are free to do whatever they want with Willow's sexuality. It's just that, in the later seasons, it felt like they were trying to pretend that her crush on Xander & her relationship with Oz never happened. I think there should have at least been some kind of scene where Buffy or someone asks her how she reconciles being a lesbian with her previous straight attractions. This is TV after all. We learn things through dialogue!
2. I dunno. I think Adam was a pretty lame villain. But I think Season 4 is highly underrated due to many of the show's greatest stand-alones, including "The Freshman," "Living Conditions," "Fear Itself," "Pangs," "Something Blue," "Hush," "This Year's Girl," "Who Are You?," "Superstar," & "Restless." Even a really crappy episode like "Where the Wild Things Are" still has tons of great 1-liners from Xander, Spike, & Anya.
3. Boo for all of Joss Whedon's gratuitous character killings. At least Marvel will make sure he doesn't kill off anyone too important in
The Avengers. (But I have a sinking feeling that Agent Coulson should make sure his life insurance is up to date.

)
4. Agreed. I loved Spike as a villain in Season 2, as comic relief in Season 4, and as a hilarious double-act with Angel on
Angel. But I have absolutely no interest in Spike as a lovelorn romantic hero in Seasons 6 & 7. Part of the problem is that I don't think there were any characters in the
Buffy cast that Spike had any real chemistry with, except maybe Giles. (And sometimes Dawn, but I never liked Dawn.)
Ironically, I heard from James Marsters that Joss Whedon never really liked Spike and always resented the fan & network pressures to increase his presence on the show. (Marsters also sounded quite bitter about blaming Joss for getting
Angel cancelled by pressuring the WB for an early answer about whether or not they got renewed.)
And this is a big one, what's up with season 6? It almost feels like a spoiled petulant child who got bored with his toys and decides to break them all. Did anything happen behind the scenes to trigger this?
Not sure if you know it or not, but season 6 was when Buffy switched stations/channels from WB to UPN.
It could be a factor. I'm not sure I've ever heard any of the
Buffy writers comment on it. But on the
Roswell DVDs, the writers there talk about how UPN was much more hands off than the WB. The WB always had notes about wanting less sex, more action, and tended to have very specific ideas about how to maintain the likability of the characters. UPN just let them do whatever they wanted.
Maybe, maybe I wouldn't be so bitter about it, but Tara had to be the blandest most useless main character on the show(Dawn comes close mind you). What made so many characters on Buffy interesting was that they made mistakes and had human weaknesses. Tara was too perfect and just didn't fit the rest of the show(Riley sort of had the same problem).
The problem with Tara was that they took Willow, the most timid character on the show, and tried to make her the dominant one in the relationship by creating the mousiest love interest possible.