Well, the maya GUI is actually a one big MEL interpreter. Every time you issue a menu command, change an attribute, drag the timeslider and whatnot you're actually executing a MEL command. So everything you do manually in the GUI can be done via MEL scripts also. With MEL you can even create your own GUIs. Take a look at the MEL command reference in the online documentation and you'll find a huge amount of commands. Also take a look at the scripts available here at SM or for example highend3d. That should give you an idea what can be done with MEL.

I have written a little MEL tutorial. You can find it here: https://kotisivu.mtv3.fi/karppa/pier.html