It's like an avant-garde version of Turn Left, with the concept taken so much further, it reaches the edge of the universe, loops around, and comes all the way back down to Earth.
Did the episode ever explain why everyone runs away in horror, quits their job, and starts avoiding Ruby when they talk to that old woman, including trained UNIT agents? Maybe we weren't meant to know.
I also found it amusing to think about how little happens in the prime timeline. The ultimate result of this entire alternate life is that Ruby has a weird, but brief, supernatural experience, the Doctor narrowly stops himself from breaking a fairy circle, and they both have a normal day in Wales.