13. I call this the Highlander Principle of Software Package Management: There should be *one* package manager on your computer, and it should manage *all* of the installed software on your computer.
This is, IMO, true for all computers no matter what, but it is especially important for servers. If you have to run more than two or three commands to get *everything* updated with the latest security patches, that's too much work and you're going to fall behind on it.