Can I undo changes made via "defaults write"? I've been messing with Bluetooth audio settings to try to keep my Bluetooth speakers from popping by changing settings via defaults write, but I'm afraid I've made the problem worse.

I'd like to revert to the default… defaults, bringing them back to where they were originally or, more specifically, where they would be for a new user account or new system.  Is this as simple as using defaults delete on the appropriate domain and key?
Yes, usually you can just use defaults delete

If for example you have run 

defaults write -g NSScrollAnimationEnabled -bool false


You can undo the changes by running one of

defaults delete -g NSScrollAnimationEnabled
defaults write -g NSScrollAnimationEnabled -bool true


(In all cases you need to restart the application to apply changes)

Even if the preference key had some value before you used defaults it shouldn't matter if you delete the key. 

It is generally safe to delete preference keys or even complete files, 
and they can usually be recreated by OS X or applications.

The property lists modified by defaults are stored in directories like 


~/Library/Preferences/
~/Library/Containers/com.apple.mail/Data/Library/Preferences/
~/Library/Preferences/ByHost/


You can edit a plist in a text editor after converting it to XML with 

plutil -convert xml1 FILENAME