Traditionally, we fear the monsters that the Doctor does. That's why when Ten made digs at the Sontarans, we saw them as comedy characters, and not villains. It's also why the Daleks constantly return; every fairytale needs a good old fashioned villain. The Shakri were built up as being a race that the Doctor had heard about from stories when he was a child. He'd heard how ruthless they were, the pest controllers of the universe. And forty minutes of 'The Power of Three' had been dedicated to setting up the Shakri's evil plans. Imagine the surprise of just about everyone though when the Shakri disappeared into thin air. Not only were the Shakri massively disappointing, their plan was foiled by the Doctor with a few waves of his magic screwdriver. This was built up as one of the most complicated and intelligent threats since the revival, and yet, the alien were wasted in what was essentially a cameo in an episode all about Amy and Rory. A lesson to any Doctor Who writer; if you're going to introduce a brilliant villain, at least give them a satisfying arc.
