I am really, really excited about this series (more than usual) for one simple reason: I don't really know what to expect from the story arc.
The last three seasons I knew who the main old-series villains were going to be in each season finale - Daleks, then Cybermen, then The Master. They're the Big Three enemies of the Doctor Who world, so it was obvious all the way they were going to be the antagonists.
In Series Four, I have no idea who is going to be the main baddies! The Sontarans are good for a one-off, but they won't stand up against any of the Big Three.