From doing more testing and online reading, I can't demonstrate some of the features on my list (which came largely from "Learning the bash Shell" by Cameron Newham & Bill Rosenblatt) but there are other features that aren't on the list above that work. readline (and bash and the mongo shell version 1.8) accept Ctrl-_ to undo a single change, and this can be repeated to undo multiple changes. So, you can press the Backspace key 20 times (which deletes text, does not kill it, so that it could be yanked), and then press Ctrl-_ repeatedly to bring back all of the deleted text. This works under Gnome Terminal from Ubuntu 11.10 with both bash and mongo 1.8.2. readline also supports numeric arguments for "commands", including the command of inserting a character. So, you can hit Esc then 5 then 'g' and five 'g's will be inserted. You can also hold down Alt and type digits to specify the numeric argument. Negative arguments are also legal and work with some commands (but not all). Typing Esc - 1 0 <right arrow> will move the cursor to the left by 10 characters, but Esc - 3 Ctrl-W kills to the left and does it a single time.
I was wrong in my comment from Dec 08 2011 01:15:53 AM UTC that readline doesn't support Meta-Y to rotate the kill ring. I must have been testing in the wrong terminal. It won't work for me in Ubuntu's xterm with default settings, but it works fine in Gnome Terminal using mongo version 1.8.2.
It will be a judgement call how many of the more advanced features make the cut as "key" features. Kill and Yank are must-haves as is Ctrl-R search; if you have an opinion about what is vital and what you would never use, add a comment to this ticket, thanks!