I've set up view options so that "message list aggregation" is set to Activity by date, threaded. This sorts things according to a collapsable date group. The default is that when navigating to a folder, emails that are so many days old are hidden, collapsed under the old date. I'd prefer for all groups (days) to be expanded by default. Any way to get this?

Weird. I just found the setting which, I swear, was not present in earlier versions of KMail. I thought I would dig around again and, voila, there it was (under "Default Aggregation -> Configure -> Group Expand Policy" in Appearance settings, Message List tab).