Synaptic / Apt: Check for unused packages?

[Chanchao]

I was doing a dist-upgrade of an older machine from Breezy to Dapper
yesterday (won't be doing that again. :) and I noticed a lot of older
packages/libraries were actually being re-installed.  The one that I
noticed was GStreamer 0.8, where the current version is 0.10.   The only
reason I noticed this is because I have the experience with the
GStreamer version numbers because of messing around trying to get
RestrictedFormats to work.  But who knows what else is on there double,
what older packages might be still around..

So, is there a way to use apt / Synaptic to check for packages /
libraries that are no longer used by anything?  Does apt even
distinguish between applications and libraries?  I think it should,
because otherwise there would be no way to tell what is a top level
application and what is a library.  For sure when nothing else depends
on a library then it can be removed. 

This dist-upgrade took such a long time (both downloading and
installing, then the install process sometimes required user-input so
you have to mother over it all the time), plus I saw all kinds of error
messages, plus then X/GDM tried to start but failed, HUGE amount of
hassle.  It would have been quite a bit faster to just download a Dapper
installer and then quickly apt-get the application I had before.  And
then I wouldn't have that worrying feeling that there's all kinds of old
crap still around.  I got rid of Gstreamer0.8 and a whole lot of old
kernels manually, but what else is in there that I don't know about.. 

Just the installation/configuration took 6 hours altogether, that was
AFTER the downloading last night..

Cheers,
Chanchao