Aptitude package management guide for control freaks:

* The aptitude pattern '~E | ~prequired | ~pstandard | ~pimportant'
  gives the "base" Debian system.  You should have hardly any packages
  listed when you group that expression and add '!~i' (not installed):

  aptitude search '(~E | ~prequired | ~pstandard | ~pimportant) !~i'

  (Possibly, some packages that were standardized, made important
  etc. after you installed your system will appear.)

* Negating said pattern (which is equal to '~poptional | ~pextra')
  gives all non-base packages.  Add "~i" to get installed non-base
  packages, and further add "!~M" to exclude dependencies (~M means
  auto, not manual; don't ask).  You should end up with a relatively
  short list, which includes non-base stuff you installed explicitly,
  and non-base stuff that was installed for whatever reason; you can
  pick out stuff from here to remove; mark them as auto-installs and
  run 'apt-get autoremove' to safely remove the unneeded ones.

* Apt's default configuration will not autoremove a package when it's
  "suggested" by a currently installed one.  This can be changed by
  creating a file in /etc/apt/apt.conf.d, say "99suggests", containing
  the line 'APT::AutoRemove::SuggestsImportant "false";'.

  WARNING: Some very important packages, like grub-pc, are not in the
  "base" system mentioned above, and might get autoremoved after doing
  the above if you don't mark them as manual installations.

* If you keep a list of explicitly desired packages, my-packages.txt,
  you can check your system against this list the following way:

** Mark all "base" packages as manual installations:

   aptitude unmarkauto '~E | ~prequired | ~pstandard | ~pimportant'

** Compare that list of packages plus your explicit package list
   against the list of manually installed packages:

   apt_list(){ aptitude search -F %p --disable-columns "$@"; }
   
   base='~E | ~prequired | ~pstandard | ~pimportant'
   manual='~i !~M'
   
   diff -y --suppress-common \
        <(apt_list "$base" | sort -u - my-packages.txt) \
        <(apt_list "$manual")

** In the output of the above, the left column will show packages you
   desire but don't have installed; the right column will show
   packages that you may want to remove; mark them as auto-installs
   and run 'apt-get autoremove'.

Note: sometimes after you've run 'apt-get update' and some "base"
package has a newer version but you still have old one installed, or
don't have configuration files of the old one pruned, or it's still in
your database for some other reason, then it will pop up in the "base"
package list explained in the first point.  Run "apt-get dist-upgrade"
and "aptitude purge '~c'" and "aptitude autoclean" to prevent this.

This guide provided to you by a patient of OCPD.  (... not really.)