Joss' (and other writer's) commentaries on the DVDs are making perfect sense; I can't exactly tell which episodes he commented on but over the seasons it goes along those lines:
It isn't thrilling if nobody is in danger, but we can't threaten Buffy that much, she is the main character, nobody would believe she could actually die .. so we put the others in danger.
Willow is perfect to be put in threatening situations, Aly plays the emotions so well and people can identify with her.
Willow has become such a strong character over the last years, so she isn't a little girl anymore, who we can put in dangerous situations, we needed someone new. Tara's character was perfect for that, we really planned to make her like the early Willow. [That's on the Hush commentary]
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So there, maybe I should take the time one day to search for the exact quotes, but this makes it even clearer. Joss wanted people (Scoobies) in life threatening danger. But Willow got to strong and the only reason Tara seemed to be introduced was to make people like her, identify with her and then.. well, you know what happened.
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Speaking about widescreen .. I noticed that the scenes are badly cut. Which doesn't matter on TV, but on the widescreen DVDs you see more of the left and right part of the picture, and often there are people moving there mouths, talking, but with no sound. I just saw This Year's Girl, where Buffy and Willow walk across campus, and just before they bump into Faith you see Willow talking and Buffy listening.. but no sound. The same happens between Willow and Tara with Tara supposedly out of the frame in Superstar, when Tara is about to take her "exit". This sucks! And I hate the stretchy look.
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Buffy: Kill the bad fairy... destroy the bad fairy's
powercenter, whatever, and all the troubles go away? ...
World is what it is. We fight. We die. Wishing
doesn't change that.
Giles: I have to believe in a better world.