Currently, the default sort order for Activities on Related Lists (Activities under Accounts, Leads, Contacts, Potentials) is by Created Date. I don't believe this is helpful.
I propose that the related lists for Open Activities be sorted by the earliest due date first (ascending) and Closed Activities sorted by most recent due date first (descending).
I understand that sorting by due date is currently available, but since this is not by default and must be done manually, the probability of an Activity being missed is higher.