What happens when everything goes wrong at once?
This is a question that many people - perhaps almost all people - will face at some point in their lives, when not just one thing goes horrendously wrong but suddenly multiple problems crop up together. Work, relationships and family problems become insurmountable, leading to unhealthy coping mechanisms, addictions, exceptionally poor relationship choices and a failure to communicate effectively with friends.
Buffy the Vampire Slayer always worked most effectively as a metaphor, using demons to stand in for school bullies or a magical threat to the world as a stand in for academic stresses. In Season 6 the show still does that, but it also takes a surprisingly deep dive into real stresses and anxieties. Sure, the root causes of the stresses our characters encounter are supernatural - such as Buffy's PTSD after suffering horrendously in the Season 5 finale - but the ways they cope are remarkably ordinary. If Seasons 1-3 of Buffy were about the traumas of high school and Seasons 4-5 were about college life and starting to realise your own place in the world, Season 6 is about becoming an adult and suddenly having real responsibilities land on your lap, and the realisation that you are in control of your own life and should really try not to mess it up.
It's bold move for a show that previously mixed real life issues with metaphors, comedy and romance, and like most such bold moves it was (and remains, seventeen years on) very divisive among fans. People who tuned in to Buffy to see the Slayer kick ass, Xander make a funny quip and Giles get exasperated felt uncomfortable instead to see their characters having a miserable time of it and making one horrendous mistake after another.
This is exemplified by the annual external threat, the traditional "Big Bad," which in fact is pretty risible for most of the season: two very minor antagonists from previous seasons team up with a dude no-one remembers to take over Sunnydale. Their plans are mostly stupid and go wrong, and their actual threat value is somewhere around zero...right up until it isn't and then things go south very fast.
For most of the season, the show is more concerned with the long trip through the soul of Buffy Summers. Suffering severely after the events at the end of Season 5 and having to take care of her little sister and keep their house running, Buffy is also forced to quit college and get a horrible, soul-destroying job to keep the money coming in*. Buffy's problems are startlingly mundane and her reaction to them - internalising her stress, trying to keep a brave face on as everything comes crumbling down inside - is both natural and leads to horribly predictable coping mechanisms.
What's even worse is that our other characters are unable to help because they are also suffering their own problems: Xander and Anya have relationship issues, Willow's growing addiction to magic is causing problems, Spike doesn't know what his purpose is any more and even Dawn is making poor choices, whilst Giles is trying to work up the courage to leave Sunnydale for good. The Scooby Gang has become completely dysfunctional apart from the perennially unflappable Tara, who finds herself - not entirely comfortably - becoming the confidante of the group as they struggle to deal with their issues.
The season does handle its more challenging and realistic material well (especially since you can now blitz through it in a few hours, rather than over nine months), but there are a couple of problems. One is that Willow's "addiction = drugs" storyline is a bit too silly and on the nose, literally sending her into a 1960s-style trippy den of inequity at one point. It's at that point the subtext becomes not just text but an entire novel filled with the words "DO YOU GET IT YET?" Mercifully the show quickly gets a grip and moves on from that idea.
Buffy the Vampire Slayer's sixth and penultimate season (****½) is generally regarded as the darkest season of the show, which is definitely true. It's also the season that has aged the best since the show originally aired. It's the psychologically richest season of the show, the one which really gets under the characters' skin and explores them in a manner the show never managed before. I also suspect the twenty-somethings who were irked with the season's darkness because they were having a great time may be much more sympathetic to it as forty-somethings with baggage (cough). The season is available now as part of the complete Buffy the Vampire Slayer DVD box set (UK, USA).
* Exactly why the Watchers' Council aren't picking up the tab, given Buffy and Giles's reinstatement in Season 5, is curiously unexplained.