I think they did this because ratings were steadily sliding and the show's peak was in "the high school years", but it works thematically - Dawn's maturing and her life is going in a newer, harder direction (i.e. High School), and Buffy, as an adult, now, still has ties to that young, innocent child within.

One of my favorite moments in the series: Buffy scolding Dawn for dancing with RJ at the Bronze (and admittedly, lying to buffy about going to the library) and flashing back in my head to Buffy sneaking out of the house in S1 or dancingat the Bronze in School Hard, WSWB, Bad Girls. Such a perfect older/younger sibling moment.

the beginning run of that season (aside from the meh "Lessons") is one of the finest of any season IMO. FTR, I loved the contrast between the old, dark SDHS, which looked very much like a the middle school I attended that was about the same age as SDHS supposedly was, and the brightness of the new building.

this is Buffy, as a maiden and a warrior, but her soul remains the same - it's the same look in her eyes, fear and determination.

THIS, 150%. Just one of the most perfect descriptions of her I've ever read.