List all manually installed packages

The main task of a package manager is to keep track of the list of packages the user wants to have installed and to automatically deal with their dependencies, making sure that exactly those packages are installed that depend on the requested ones. Unfortunately it is not even possible to display that list. There exists a filter for all automatically installed packages, while exactly the opposite would be interesting. In short: I think it would be very useful to include the functionality of:

aptitude search ~i | grep -v "i A"