goldenusagi posted a question about Dawn's role and character development, whch has inspired the following short thoughts from me:
In Season 5 the whole world revolved around Dawn. She literally carried the fate of the universe within her veins. Hellgods and ancient mystical orders were chasing her. Her sister and her friends were totally devoted to protecting her; she was their number one concern. Dawn's personal issues - her need to believe she was real, that she had a family - were taken seriously by everybody.
In Season 6, none of that was true anymore. Not only was she no longer the centre of attention: people seemed to be actively ignoring and avoiding her. Buffy died, and when she came back she seemed to have no love or energy to spare for her sister. Willow and Tara, who had become her surrogate parents in a way after Buffy's death, had problems of their own -- and then Tara walked out and Willow tried to kill her. Spike spent all his time chasing after Buffy and forgot about Dawn. Even Dawn's first boyfriend turned out to be a vampire and had to be slain.
It's no wonder she felt abandonment issues. In response, she started acting out, trying to get noticed. She started stealing things -- partly, I guess, out of spite: if they were going to ignore her she might as well take advantage of that fact; and partly out of a subconscious desire to get caught, because having people angry at her would at least be a step up from having them ignore her.
By Season 7, most of Dawn's personality issues had been sorted out. She no longer doubted she was real, that she had a place in the world; and once Buffy recovered from her depression Dawn once again had a family and people who could show her that they loved her. In Season 7, by consequence, she's much more well-balanced and likeable. Her main issue now is that she wants to be respected; she wants to be taken seriously as an adult. She's eager to prove she can help, and resentful of any attempt to shunt her off to the sidelines.
Describing Dawn's character arc in Season 8 is more problematic because it's still in progress, but I'll take a stab at it. She's now an adult for real, independent and able to make her own decisions -- but she screws that up. In her own eyes she makes a mistake, does something wrong, something she deserves to be punished for. She's simultaneously resentful of her sister for not being there to help her, but at the same time rejects all attempts to offer help because this is her own problem to deal with, no-one else's. Eventually she is able to move on -- not, disappointingly to many, by shifting the blame for her difficulties to someone else (however justifiably), but by putting the whole episode behind her and getting on with her life. In the later season she's pro-active, forward-looking, even giving signs of growing into a leadership role, and is fully integrated into her social circle -- but still, to her annoyance, not always taken seriously enough by those older than her.
In short, while Dawn was no longer the focus of a season arc after Season 5, the writers actually integrated that into her character development, and gave her a consistent pattern of growth which was reflected and built on in the episodes - like 'All the Way', 'Wrecked', 'Older and Far Away', 'Lessons' and 'Potential' - which were Dawn-centric.