And packaging, of course
Packages are essential to the long-term stability of a system. Knowing what you've got and how it got to be there are essential.
It's vital not to be distracted by the ".deb vs .rpm" "debate". They're equivalent. With one *possible* exception, anyone telling you that one or the other is better simply doesn't know how to drive the one he's slagging off.
When I was getting to know Linux, I broke a number of installations by relying on the advice of people who told me just to "make install" eveything. It works - for a little while. Then you find out you've over-written something that another application relied on...
 I use the term quite wrongly, of course.
 I'm not sure it's possible to find out how a package's files have changed since installation in the apt/dpkg world; the equivalent of "rpm -V". This might just be ahole in my knowledge, but I didn't get any answers when I asked the LUG...