Having a bit of a play with an old idea.
I have a thing for characters that have a single, almost uni-directional motif or desire. It encourages whatever situation they're put in to be approached in an interesting way - doing whatever it is they can regardless of how hopeless, naive, ineffective, straight to the point, effective and easy, unable or great it may be, it will be interesting. Unlike, say, Superman. Who can do anything. Which is crap and boring.
It also makes the characters easy to relate to because of how simple they are to read and how quickly you can come to know them. A good, clear, entertaining experience for everyone! For people watching or playing with them and the people working with them.
.. hey look at that - I did words!