ITA that if Dark!Willow had been around for much longer, the conception of her would have to have been quite a bit different. You really can't keep up "Witch Smash!" for much more than that.
When I read those foilers, I had hoped that the season would be ambiguous in a different way. That Willow would have gotten involved with the Trio or something else bad, but that it wouldn't clearly be bad, and bad from the beginning. As Fred points out, as early as All the Way, we knew she was doing very bad things, because she mind-wiped Tara. Even as early as (I think) Flooded, we knew she was gone with the "Maybe you'd better not piss me off," comment to Giles.
I actually really enjoyed season 6, it's at least in my top half of seasons of BtVS (I think).
I liked S6, loved the Dark Willow arc, got way too much enjoyment about her beating on Buffy, and thought the end run of episodes totally redeemed the fallen souffle of the middle.
To me, Willow's Buffy smackdown didn't make much sense, so it didn't work. I think it could have. Throughout the series as a whole, I think there have been plenty of storylines that support Willow's dissatisfaction with being the sidekick, but it didn't fit where they put it, and how they put it. She brought Buffy back from Heaven. I don't think she ever even apologized or tried to talk to Buffy, she just tried to mindwipe her. For her part, I don't think Buffy ever even got blame-y with Willow. As far as we can tell, she mooched off of Buffy, as did Tara (although Tara pulled her weight by being the only actual grown up on the series in S6, and I include S6!Giles in that indictment).
Looking back, I don't think it was the Actual Car Crash that did the story line in. It was de-ratting Amy. Partly because of making everyone disappear at the Bronze (though that fits in with the power trip), but mostly because Amy introduced Willow to Rak.
I wouldn't have wanted Buffy to be vamped in Season 7, but I would have enjoyed her being bad, in a less metaphory (the FE) way. I feel a similar dissatisfaction (although for different reasons) with Willow in s6. For all that I thought the booksuck, and flaying were exciting, and the smackdowns with Buffy and Giles were hurty (which is good, in that we want to have reactions to these things), and for all that the breaky yellow crayon speech and obligatory!Sarah-Song made me cry, when I look at it now, most of the final two hours just feel like camp, and not good camp.