When you click on Protect Document, one of the options refers to "Formatting
and Editing." Clicking on this one opens the same Protect Document task pane
as in Word 2003, where you can select "Editing restrictions" and then
"Filling in forms."
Unless you need to protect specific sections, however, you don't really need
to use this task pane. You can instead use the same padlock button that used
to be on the Forms toolbar. You can add this button to the QAT. In the
Customize dialog (All Commands), it's the one with the padlock icon that
just says "Lock."