Take Xander. My husband and I were discussing Xander and Willow yesterday. (He, somewhat predictably, is a huge Willow fan.) We were discussing the thing I had just read in the wikipedia article on Xander, saying that Whedon had set up hooks for all the main characters to come out at some point. He wanted one of them to be gay, but hadn't decided which. I said that I really wished Whedon had picked Xander and not Willow. Willow had two things going on with her character, and Xander had zero. Making him the gay one would have given him one thing of interest, one hook for interesting development in canon.
No, I don't count Anya as an interesting development in Xander's character. I count Anya as a very funny joke that was stretched out way way way longer than it ought to have been.
I don't particularly care one way or the other which sex Xander prefers to sleep with. I just felt horrible watching the character slide into uselessness in seasons four and five. I like early Xander. And I got very very tired of addicted-to-magic Willow. So I feel the need to do something interesting with Xander. I wanna give the boy a character arc.
And I'm trying to invent a consistent magic system, to make Willow's story less boring. (The SF writer can't help but have coherent rules behind things. This is how we know that canon-Buffy was written by romantics, not SF writers.) I think it's more interesting if the danger in magic is in the lure of power, something that grabs at the character, and not in mere physical addiction.
As for Buffy... well, I started actively loathing her in season 4. I liked her again for a while in season 5, then was a little put off by her ultimately succesful descent into suicide. She really didn't want to find any solutions to the Glory problem that didn't involve her own death. And then it got worse. Poor Buffy. I loved the Buffy in "Welcome to the Hellmouth," the one who saw Cordelia picking on Willow and winced. The perky bright optimistic Buffy, who hated her destiny but threw herself into life anyway. I think Buffy can be interesting without being broken, without losing her drive and optimism.
And of course Giles is an utterly fascinating character, set in place and then not really used by the canon screenplays. The little of his history we know is intriguing. We can take it in many directions, all plausible. The character traits we see are interesting, and we can explain them any way we want. Mutant Enemy's omissions are our opportunities. But I don't need to tell you that, dear reader. You already love Giles.
And I do prefer to pretend that the pit of bad writing known as season 7 just never happened.