When logging a call, note or email on a contact file if there is a task on the file the "log call" button will automatically be "log call and complete task". This is not always appropriate (e.g. if I am logging that the person did not answer or if I am adding a note as to why the task could not yet be completed etc)
I note when sending an email you get a drop down option to send, send later, or send and complete task. This would be appropriate to have as standard for all activity types, and have the user set the default they use the most.