How many times have we heard, "My sister stopped using Ubuntu when she had to do something with the command-line/terminal" or "My girlfriend doesn't find Ubuntu usable because of the things you have to do using the command-line"? (Edit: No sexism intended. Please read my comment about that)
(If you haven't then read about the Great Ubuntu-Girlfriend experiment :
http://contentconsumer.com/2008/04/27/is-ubuntu-useable-enough-for-my-girlfrien d/ )
So my suggestion: allow the community to compile a list of actions that can only be done on the command line and would be very useful if it had a UI.
This way, people can't make general statements like, "Ubuntu requires too much command-line use for newbies to use it, lets get back to MS Win".
LET US CHANGE THAT TO:
"Hey, it's surprising that you need the command line to do this (provide explanation) in Ubuntu! Please do implement a GUI for this! Thanks!"
Developers can set targets for each action and then we'd know that by Ubuntu XY.10 or XY.04, user's won't need the command line for these actions.
A great measure of progress would be to see the actions that still require the CLI and see how complicated and not "everyday actions" they are!
Hope you like the idea!