If you are, like me, an Ubuntu user who is annoyed when doing aptitude full-upgrade because it stops and displays the changelog until you press "q", then here is why this happens, and how to change the behavior.
The reason this happens is that the package apt-listchanges is installed on your system. This package lists the changes before applying them.
However, by default, it stops the upgrade process until the user has provided input. The silly part is that there are no choices at this point, the user must dismiss the output and nothing else can be done.
Sometimes you start an update in a terminal GNU screen window, then switch to another window thinking that the update is being done, while it is just stalled waiting for meaningless input, only to find that the update has not been running at all.
To change this behavior, you can edit the file /etc/apt/listchanges.conf, and change the following line:
Now, doing aptitude full-ugrade will not pause for use input.