I think the problem with the writing for Dawn was that there was a disconnect between what we were told was the other characters' level of investment in her--as far as they could remember, they had known her their entire lives and had a whole lifetime's history of memories and emotional attachment to her--and what the audience viscerally felt was OUR investment in her, because she just showed up on our screens and we were expected to respond to her not as if she were a new character, but a character who'd grown up along with Buffy and the Scoobies. But the show had never cultivated that investment in us the way it had with the other characters, through us getting to know them from S1-S4. In retrospect I can enjoy her more as a character and as a storytelling device, but I think at the time much of fandom rejected her because of this, because she was supposed to be important to us but the show hadn't bothered to make her important to us.