Say I start writing the name of a class lets call it "MyClass" then it would be nice the when I have writen just part of it parhaps only the first or two first letters to see an option of autocompletion.
So when I have writen "My", "Class" is displayed behind the caret in a way that shows it is not yet "in the code"
and if I then press say tab it completes the code.
It I would want the whole list of code completion i could still use the ctrl-space.
I'm sure not everyone would like this so it sould probebly be an option...