As I'm sure you guys have figured out from the Mean Girls survivor, Dark Willow is one of my least favorite characters.
First, she stole the Trio's thunder. Not since Angelus, Spike and Dru had I been excited about a Big Bad. They were getting everything they wanted all by using their brains. Not even Buffy could defeat them.
Second, she is instantly redeemed because it was all out of grief? She killed not only Warren, but Rack, who may have not been the most pure guy, but was a human. I thought the golden rule was not to take a human life. Not to mention all the torturing of her own friends. The Scoobies treated Spike like he was dog poo under their shoes for years, even after he started patrolling with them and occasionally saving their lives. Certain Scoobies couldn't even forgive Angel once he came back from the hell dimension (let's not EVEN talk about before that with the "Willow says kick his ass!" Screw you, Xander!)
Finally, and I know this doesn't have to do with Dark Willow per se, but she almost ends the world because she's lost her true love, and then they hook her up a couple episodes into the next season?! With someone who is completely a mockery to Tara's memory? Sorry, I can NEVER forgive someone for making me watch Kennedy more than one episode.