As we know, Ubuntu comes with a lot installed on it. Many of them I either don't know what it does or just don't use. Which applications are safe to uninstall? I use mainly the basic stuff, FireFox, Thunderbird, Empathy, Shotwell, document and photo viewers. I don't think I need something like xDiagnose or xTerm
You should use the Synaptic Package Manager or Software Centre to remove the preloaded apps and check you are not removing anything desirable along with the program you do not need. Either program will prompt if anything other than the program in question is to be removed.
i.e. removing either of the' xdiagnose' or 'xterm' packages will also remove the 'ubuntu-desktop' package. 'ubuntu-desktop' is a required package if you ever intend to upgrade to a new version of ubuntu, so removing this is not desirable.
On the other hand, if I wanted to remove the 'shotwell' package I can do so without affecting the rest of the system. The same goes for firefox and most of the other built-in apps available from the Dash.
Just remove the app unwanted apps from the menu contents. There is menu editor for this. The reason for this is that you're not going to save a massive amount of disk space and when an update of the desktop package is released (happens every now and then) and installed, all your removed applications will return.