That's an easy one :-)Terminal, with a full ability to use apt-get to install and update packages, without the hacky solution of disabling write protection on the system.
Convergence, to me, is less about being able to connect a display, mouse and keyboard (though that would be nice), and more about having the same unfettered access to the command line and traditional linux package management as I do on the desktop
FWIW, I just installed this for the first time, but the first thing I did was research how to get apt-get working Among users of a Linux phone, I imagine there must be more people like me