My opinion of the seasons from best to worst go this way:
Season 2 - best
Season 3
Season 5
Season 7
Season 1
Season 6
Season 4 - worst
My reasons for season 6 being second last? A few.
1. I felt that Dawn was underused (and I know the people who hated Dawn will argue this) but my problem was that she just sort of hung around with no purpose and that contributed to the writers having her act like a total brat. I really liked her in season 5, (especially since she behaved in my opinion more maturely in this season), but after she was no longer 'the key', she suffered from what I like to call 'Riley syndrome', meaning that her plot was no longer relevant to the main arc so they had this character left over with no main place within the scooby gang.
2. Lack of Giles. I realize that was because Tony Head wanted to return to his family in England, but it still left a gaping hole in my opinion, especially since they never really mention him either. Even when Xander and Anya have their non-wedding, they don't even say that Giles sent his regards or even called....
3. The nerds. While they were funny and I get that they were meant to underscore the fact that life can be scarier than any supervillain, I felt they were a bit of a let down and that they left the main story arc to drift in some cases.
4. Willow's 'magic as a drug addiction' storyline which I felt was one of the lamest storylines the show ever did (aside from the Initiative). In previous seasons the show had done storylines that were meant as metaphors for larger issues (Buffy sleeping with Angel being a metaphor for sleeping with the guy who doesn't call you the next day for example) but in this case the subtext just simply became 'text'. Willow all but joined 'magicks annonymous' and started a 12 step program. It was WAY too overt for a show steeped in metaphor. One of the main problems with that is the fact that addicts will ALWAYS be addicts. An alcoholic, once on the road to recovery can never ever drink again. It's too dangerous. (And having relatives who have been alcoholics I can say this is pretty true). However, Willow not only 'overcomes' her addiction, but in season seven has her friends getting angry with her for being reluctant to use magic. I think if they had played it more as an addiction to power, then it might have worked a bit better, but unfortunately it came off more 'after school special' and that really soured me on the season.
5. Xander leaving Anya at the alter. I don't know what idiot in tv land decided that married couples on tv are boring, but the fact that Joss chickened out with this marriage storyline bothered the hell out of me. I thought that of all the characters, Xander was the one most ready to take on the responsibility of marriage and it seemed like it was actually the writers who got cold feet, rather than the character. It was a real disappointment for me and I felt it really undermined his character.
I won't address the Spuffy/Bangel stuff here because that honestly doesn't factor into my opinion on the season. I am a Bangel, but I can honestly say I found the Spike/Buffy relationship interesting to watch (even if I don't think they should be together) which is more than I can say for the Buffy/Riley relationship which just made me gag anytime they were on screen.
The thing that redeems this season from being the worst (in my opinion) is the way it treats life as being one of the hardest things a person has to deal with...and anybody who has ever gone through a particularly difficult period in their lives can relate to the way each of these characters tries to deal with it (Buffy by sleeping with someone she hates so the can feel something and working a shit job, Dawn with stealing, Willow with becoming addicted to a substance, Xander with abandoning his responsibilities out of fear, etc.)
So while I do think this season has definite worth to it, not to mention some amazing episodes (like Tabula Rasa, OMWF, etc.), it's not the highest on my list....
-Sara