Since upgrading from 1.8.3 to 2.0, the Mongo shell tab completion functionality has changed. Previously, it showed you all the available commands. Now it doesn't show anything, it just completes the command. This is good if you already know all the available commands, but if you do not, or do not need to remember them, you're at a loss now. The previous functionality was better. Also when new people are trying out MongoDB, they will (at least I did) try the tab key to see if it works. It's better if it shows all available commands so people see whats available.
SERVER-4312 Add "key" missing readline/bash (EMACS-style) command line features to the shell