In this scenario, I have a user returning to form and pressing an edit button, then the form is editable a modal window. Should the user have to hit the save button? Any downside to auto-saving after a text field has been changed?
Yep, they should probably hit a save button.
With a lightbox, the only way of moving to the next task is by clicking the X in the corner. This is generally recognised as a 'dismiss' or 'cancel' action rather than 'save'.
In a lot of cases, having no save button is likely to make the user ask "Is my data being saved? ...maybe I should look around for a save button just to make sure" so having the save button is likely to avoid unnecessary mental work.
Big systems could also struggle with the extra server load of autosave.
See also this tangentially related question, What's the rationale behind Google's ''no-save'' approach.