Backup, what system files are *really* important?

[Stuart McGraw]

I currently backup user only my user files with the 
understanding that if I lose important system files, 
I will reinstall the system from distribution media.

Yesterday I had a little accident when I accidentally
deleted an unknown number of files in /var.  But 
since the rpm database was still intact, I was able
to "rpm --verify" all my packages, identify those 
with missing /var files and reinstall just those 
rpms.

Which leads to my question...  Are there certain 
files and directories that should be saved because
they are unusually important and can help recover 
from common problems, without backing up *all* the 
system files?  In my case above, having the rpm 
database was critical.  I backup config files in 
/etc and elsewhere that I've modified.  Are there 
other *really* important system files I should be 
saving?