How do I reset all GSettings key to their default values?

I want to reset all keys in the GSettings to their default values; preferably using a single command, or a simple bash script. How can I get that done?

Generally you can reset one key to its default value with

gsettings reset SCHEMA [:PATH]  KEY

So you might use a bash script to do for all available keys.
Something like (pseudocode):

for i in /dir/of/keys
do
    gsettings reset <key-path>
done

Look at its man-page for more information: man gsettings

The following will reset all those settings which are "non-relocatable". That is, the ones that are stored at a standard location and hence do not need an extra path specified after them. For example it will reset all keys of org.gnome.eog.fullscreen, but none of org.compiz.unityshell:/org/compiz/profiles/unity/plugins/unityshell/. It only does so for the user executing it.

gsettings list-schemas | xargs -n 1 gsettings reset-recursively