Yeah, Dawn was kind of...directionless in S7. It's actually kind of funny, because despite her not having a specific and season-long arc, that's my favorite season with her. She develops really well from the bratty little sister to the whiny teenager to finally being this cool and very self-reliant girl who stands with any of Buffy's team.
I'm right there with you in the sense that it was a little strange that the big Dawn arc in S6 was her bonding with Buffy, only to have that kind of shunted to the side. I don't think it took away from the season or the character, but it was a little weird. If I could have lunch with Joss (which would be SO cool) that'd be one of the things I'd ask him about. Maybe they just kind of ran out of things to do with her (which is kind of the writing wilderness Xander got lost in over the last three seasons of the show).
In the end, S7 was absolutely packed with plot lines. Between Buffy and Spike, The First, Caleb, Faith, all the drama with the Potentials, the town evacuating, Andrew and about a million other things, I think Dawn sort of fell through the cracks a bit.