For me it was the nature of the angels that made them scary. You have to keep your eye on them, or they move. Very fast! Regardless of the fact that their affect is usually non-lethal. It's purely the psychological factor of the beings themselves. The fact I used to be afraid of statues and models as a kid probably helped.* It's something about the hard perfect faces and (in the case of statues) those white empty eyes. Whilst I love that spooky look now, they still give me bit of a chill. (Actually I probably like them for that reason. I'm weird like that.) And to suddenly find yourself in another time, can you honestly say that wouldn't be scary? If they sent you back far enough you could find yourself in the same geographical country, but with people of a different language and culture. That's pretty scary stuff, but secondary to the creatures themselves. Blink is probably my favourite Who episode to date I think. I'm not sure it's the best episode to show to someone as an example of Doctor Who though. It isn't a typical Doctor Who story. On the other hand it's a good scary story in it's own right, regardless of the Doctor, and that might prove a draw in it's own right. *Only the human modelled ones though. I loved animals like lions and such.