The reason for the packages ubuntu-restricted-extras, gstreamer and libavcodec not being pre-installed on a default installation are purely legal than technical. These packages are non-free and the user has to accept a licence agreement (the EULA agreement). These packages will not be inducted into the default installation in future too, unless the licence changes.
Also, the packages gimp, nautilus-open-terminal, gdebi, unity-tweak-tool, gparted and p7-zip full may or may not be needed by many users who don't care to manipulate images(gimp) or their own hard disk(gparted), etc. However, these may or may not be included in future default releases depending upon whether Canonical feels the need.