Actually, once they had introduced the concept of the Doppelgangers, I was just waiting for Amy, Rory, or the Doctor to turn out to be one.
At some point (or possibly several) the next episode will dissolve into paranoia, when people won't know who is or isn't a Doppelganger, and indeed just because we see two, and we know one is a Doppelganger, it won't mean that the other one is real.
This episode seemed a bit slow, and like it was avoiding spending money on special effects.
I wonder if this can be used to explain the 200 year older Doctor that we saw being killed in Episode 1?