It's hard to decide whether the real 'big bad' of this season was Glory, or life itself. Time and time again Buffy and her friends are faced with as many real-life problems as supernatural ones. Riley's near fatal illness, Tara's desperation to be accepted, and Buffy having to deal with a 'new' sister and the illness and hospitalization of their mother.
This is a season wrapped in sadness. When Buffy returns home to find her mother dead on the couch, its the first step towards her final catatonic state as she is no longer able to cope with the world. Her final sacrifice made it clear that this wasn't a kids show, it spanned various groups of people and there was a meesage in this season for everyone!