How can I export all my Terminal.app settings from one Mac to another?

~/Library/Preferences/com.apple.Terminal.plist contains Terminal settings.

You can view the file (if you want to) with Property List Editor, part of Apple's developer tools. Copy it to to your other machine to take your settings with you.

You can also export one Settings entry in Terminal by selecting Shell, Export Settings..., or by pressing the cog button below the list in Terminal Preferences, Settings and choosing Export....

Copying the com.apple.Terminal.plist works very well. Be sure, however, to NOT have Terminal running when you copy the file as it will overwrite it when it exits.