However, its not getting a excellent because the big bad was not only not defeated, he/she wasn't even revealled, this, unlike Rivers storyline, should have been tied up.
Why?
Pretty much every other arc-based show makes a big deal of setting stuff up for the next series... why must
Who answer all the questions in 13 episodes?
I'm refreshed to see we still have a mystery for Series Six, to be honest. And the resolution will be better for not wrapping it up in a neat little package this ep. Consider, we now know there's an enemy out there with the power to destroy the TARDIS and control it in flight. Series Six will focus on the Doctor finding out who is after him... WHY did they want to destroy the universe? Presumably they could survive the end of all things... WHO could do that?
Those are questions too big to adequately address in one series finale... better to leave them aside and let the Doctor have his small victory (of saving the whole of Creation... again).