Remember how, in Series 1's heart-wrenching episode Father's Day, the Reapers appeared out of thin air to eat us all up when time was altered? They were scary, effectively realised and a great reason for not tampering with established history. Yup, Reapers will eat you up when history is changed... except when, for no readily discernible reason, they won't. In every subsequent story wherein time has been mangled, the Reapers have conspicuously failed to appear. In fact, the Doctor has never so much as mentioned them again. The Reapers are another example of an inconvenient element being erased, because Reapers appearing each and every time history is damaged is unworkable in a show where history is rewritten in practically every episode. While on the subject of Father's Day, how stupid was the Doctor in that story? That's a special kind of thick, letting Rose get that close to her father as he's about to die. Definitely no scope for something to go wrong there. Again, let's lift up the edge of that carpet and blissfully sweep away.