I've just been watching season 6 and about to wrap it up. Here's some of my thoughts on that I don't like about Buffy (compared to the mountain of things I love about Buffy, don't get me wrong):
1. Xander wussing out on his wedding day to Anya. Then he was given a second chance to make things right and he blew it again. It basically undid all of the character's growth into manhood that had been happening the previous three seasons and regressed him back to the Xander of seasons 1-3.
2. Willow really is too cute for her own good. Several have said this above, but I too have cringed at some of her cutesy lines that sound like they came out of the mouth of a ten year old.
3. I loved season 6, until the train wreck at the altar and then the miserable and dreary Seeing Red. First, regarding Spike's attempted rape of Buffy, the show did not need to go there. It had alredy done a fine job of portraying the dynamic between these two, what drew them together, and why a romantic relationship could never work between them. The rape scene IMO added nothing and only served to give me a creepy feeling whenever I look at Spike now. Then of course, Warren randomly shooting and killing Tara was to me just a plot device so we could see Willow jump to the dark side. All season long the Trio was just a group of bumbling nerds whose attempts at super villainy were comical, then all of a sudden Warren turns into this murdering sociopath and kills off a beloved character. WTF?
3. I too hated Glory. Like someone above said, it's what happens when you give a ditzy valley girl ultimate power. The thing is, this is supposed to be a god who is older than the earth, shouldn't she be a little less like a spoiled teenage brat?
4. Buffy's mom annoyed me. Sure, there were episodes when she was fun, like Band Candy, but most of the time I was sick of her June Cleaver act, especially in the first two seasons.
5. They wrecked Spike. I loved Spike from his first moments on the show. It all went downhill when they stuck that damn chip in his head. I never wanted Spike with a soul, we had Angel for that.
6. I loved Drusilla, and I wish she had been on the show more. At least she got some time on Angel.