Basically all options able to be defaulted to any calendar. Duration (including all day), notifications (type and frequency), location, even details and the options to come (like attachments).

One calendar is "Birthdays" while one calendar is "Meetings." We can see how the defaults would be very different here yes? Every birthday is saved as all day and will want to be reminded further out. Meetings are probably short specific times and reminders are closer to the date. This is true for each and every calendar yes?

I'd say a big opportunity to increase efficiency here.