I agree about Season 3. One detail I liked was they started adding bruises and welts to characters' faces and bodies and left them uncommented on. And Seth Green surprised me. He's been acting since he was a little kid, but I've never considered him someone who could portray, for example, a reasoned response to his girlfriend cheating on him.
I also thought it was genuinely moving when Buffy's classmates gave her an umbrella and a superlative. As well as the brief, not explicitly set up shot of Willow crying on the toilet after she learned Xander lost his virginity, which was like something from Freaks and Geeks. And Angel and Faith mocking a chained-up Buffy, and Cordelia getting impaled and breaking up with Xander. That faked me out, I thought she was dead.
My complaints still hold about the kickboxing and the monster makeup, though. The worst was Balthazar, the Devito-as-Penguin guy who lived in a tub and ate people with his belly button. I wish they'd gone less-is-more with monsters as a rule. I liked Faith, Trick and the Mayor. They were all schtick-based (Faith's was city accent plus wifebeater), but they worked. The Mayor was pretty menacing. I thought Trick was killed off too simply.
But the rest of the villains did need to be stronger to sell the scenes where they're not comic relief. And sometimes the character deaths seem like "okay, for this moment, real-life rules apply," not the results of physical combat. (Kendra from S2 and Larry the Bully Who Came Out, I'm looking at you.)
The execution of the end fight needed work, but I liked the idea behind it, extending Buffy's heroism to the rest of the school. Much less ham-handed than NYers throwing bottles at the Green Goblin.