In S5 Dawn is part of the plot, the mystery, the seasonal arc.
In S7 Dawn has an engaging character arc of her own that is separate from what Buffy is doing.
In S6 she has neither. She seems to have no 'meta' reason to be beyond cast member. She is like Riley in S5; she has no life of her own, there is no sense that she exists beyond the 'Buffy's lil' sister' box. Sure, we have '
All The Way' but that fumbled the attempt by making her '
a day in the life' completely devoid of any surprises or interesting characters.
Like
@Taake indicated Dawn is very much 'what about me!!' in episodes that we- the viewer- are made to identify and sympathize with Buffy's extreme trauma. Buffy think she's killed somebody and is utterly guilt-ridden?
How does this effect me? Says Dawn.
Even worse; when Buffy is having vivid hallucinations about being trapped in an asylum, not knowing what's real and is in the grip of existential dread. Dawn's reaction?
In your perfect world, I'm not even there. Waaahhh! Yes, Dawn, make Buffy's trauma all about you.
It gets tiring after a while.
There are bits of S6 Dawn that I like, little throw-away bits like her gently treating Buffy in '
After Life' or her quiet (
non-self-pitying) realization in '
Doublemeat Palace' that Buffy will probably always have a crappy, non-special day job because she's already got that secret one that takes up all her time and effort. Urgh, I said something nice about '
Doublemeant Palace' and now I feel dirty...
Oh! Or her sweet, giddy happiness to Willow and Tara getting back together.
I honestly think Dawn needed a 'Zeppo' episode where she and her life and how she sees herself are brought to the fore and everyone else is periphery. I always liked the idea (not mine) of '
Dawn's Adventures In Babysitting' or something, that she has to solve this minor supernatural stuff herself without any big sister input. And also Buffy never finds out.
Something kinda like her (excellent) spot in '
Conversations With Dead People' but fleshed out a bit, with the other characters doing something adult, depressing and boring.
Bring Amber Tamblyn back and have an annoying kid for company that Dawn has to suffer through (so she gets a Buffy pov for a bit). And y'know a haunting or a minor demon or a goblin or something. S6 needed more levity and 'Dawn's disastrous attempt to make money for Buffy' could fit. There, that's the hook; Dawn is trying to be a good sister and make some cash for the household and now she has to fight a possessed toaster. Makes her more sympathetic too.