I already did something along the lines of...
cd /var/cache/apt/archives
ls -1 *.deb | xargs sudo dpkg -Gi
But this will install everything you installed if you never cleaned that
dir... you may find useful this command in the future:
$ aptitude search '~i'
The ~i means all installed packages. The generated list will include all
automatically installed packages so it's not perfect for your needs, but
I do not remember the exact syntax to exclude them. Aptitude's manual
should help here.
You could try to not reinstall the whole system with installer, but
simply use aptitude (with the ncurse interface) to purge (and not
remove, so that configuration files are removed and reinstalled) all
packages. Then, installing them back, if you know what you need. Given
that you still have the files in /var/cache/apt/archives it should not
involve any downloading, and you will skip the installer's questions.
Between the purge and reinstalling, you could probably change back every
ownership and perms to root in / (except /home of course) and remove
users and groups which could have been installed by packages.
It is simply a solution which might be faster than reinstalling. Or not,
it depends on what you have installed so far.