If you remember you export the list of packages? That sounds very manual, and prone to failures.

I'm surprised people in this thread aren't talking about backups. I have my dotfiles stored under revision-control, like many others here. I have a couple of notes about the setup of each host I store too. But basically I install Debian packages and that's 99% of the my install.

The only stuff that I install, outside my own code, are things like the ARduino IDE, Firefox, Calibre, and they get installed beneath /opt

Despite the lack of "real" state I still take a backup every day, and that has saved me more than once.