Thinking particularly about Rock / Punk / Metal sorts of bands, a consistent complaint is that over the course of several albums, bands seem to go a bit softer, 'maturing' or 'mellowing' or whatever.
Personally I think this is hardly surprising, given that this is a process that happens in many people's lives as well. My take is that if a band has delivered some really great music, you should just be glad you've got that, and accept whatever they want to do after that (although it is pretty gash to see an awesome band disappear up their own arses).
But - can you think of any bands whose career has followed the opposite path? Started pretty hard / fast and gotten harder / faster? I was thinking about it yesterday, but couldn't think of any. Also, sorry this is such a long post!