I actually agree with you almost entirely, Kestrel
. I don't have a problem with season 6 - in fact it was the first season of the show that I bought on DVD. I will grant that perhaps the episodes are a little too one-note - too much misery and angst all at once, rather than ups-and-downs of angst then comedy then action then back to angst, but all in all I think it was a valid direction for the story and the characters to take, it was done well.
I also don't have a problem with the Trio, aka the Legion of Dorkness. They were hilarious because you know full well everyone on this board has had exactly those same arguments over who was the best Bond or "Bazoombas! Bazoombas!" Just like people don't like Adam from season 4, even though to me he was the perfect thematic counterpoint to the Scoobies' character arc that season, the Trio is the perfect thematic counterpoint to the Scoobies' character arc this season too.
Joss always said the theme of the season was "Oh, grow up!" The characters are forced to face real life and become adults. Real life is the Big Bad. But the Trio refuse to do the hard work of growing up, of facing real life as it actually is and going through the process. They try to side-step the process altogether, using their powers as a short cut to becoming successful or rich or getting the girls. And that refusal to do the hard work of growing up leads to disaster. (Willow, by the way, does basically the same thing by turning to magic to solve all her problems, which is why she is also a villain by the end of the season.)
I don't think it's any coincidence that after vampires and demons and cyborgs and hellgods, it's a bunch of human nerds that cause the most damage to the Scoobies. Again, it's all about the theme of "real life" - facing the horrors that humans can do to each other. Buffy can't fight them in the same way she can vampires and demons - it's always been a part of the show's and the character's morality that she can't kill humans. The human world has its own laws for dealing with them, and she's supposed to only deal with supernatural threats. But in this show about magic and monsters, it's one human shooting another human with a gun (a very human weapon) that is the big turning point of the season, and that's absolutely deliberate.