It's the script
I was all set to pen a long post setting out why (see title) is the major failing of this episode, and the series to be honest. Analysis of shows where it has been possible to tell a self-contained story in under an hour, progress an arc theme and get the audience to actually care what happens to the characters (B5, Buffy, Original Star Trek for example) would have been called upon etc., etc..
Then I started reading some of the comments under a review in an online broadsheet. "OMG. Can't WAIT to see what Clara is wearing next week" and I realised that it isn't about the science, philosophy and a plain simple examination of interesting ideas. New Doctor Who has to satisfy a wide range of viewers' expectations.
What's next Saturday's episode? Monkey Tennis?