I think when someone first adds a subscription it should be very easy to tell TA that you want to get the entire channel or not or give some nice controls about which videos exactly they will be getting when they subscribe. Maybe radio buttons or checkboxes or something "Download entire channel?" and "Specify date range for which videos you want" or "Get X number of recent videos?"

The only way to do this currently is to adjust a global setting about how far back TA looks for all subscriptions.