*. "Define" days/time easily.
*. Add notes.
*. Tag organizer.
*. Search within the app.
*. It supports plugins which ads interesting features, for example, by installing a plugin called Remember the Milk, you can make GTG automatically mail you notifying about the upcoming tasks as well!.
Although these are few of the main features... if you're looking for a easy to use personal/professional task organizer, then I humbly urge you to try the GTG.
So, if you use Ubuntu, then you can easily install GTG using both the "Software Center" and the Terminal. If you're using the Terminal, then issue the below command.
sudo apt-get install gtg