Woolseyism: The German titles. The whole reason the English titles were changed was because some executive felt the two didn't go together, making it difficult for casual readers to realize they were a series. Unfortunately, the new titles make very little sense if you've read the books. Thanks to compound words, the Germans managed to avoid both problems awesomely, by calling the books Doppelganger and Hexenkrieger (essentially, witch-warrior).
One-Scene Wonder: Herbert Lom, whose character Hassler (a double agent) is quickly found out in the beginning after releasing some top secret info to the East, and subsequently is quickly killed off. He's also given leading billing (sixth) with the other main actors in the film.
They Wasted a Perfectly Good Plot: A frequent complaint about the film is that while the idea of a Counter-Earth could make a fascinating plot, the idea is wasted due to the fact that there are no differences between the two Earths other than the fact that everything is backwards.
Visual Effects of Awesome: The miniature effects in this film are absolutely superb, particularly the climactic destruction of the Space Centre. To be fair, this is to be expected from a Gerry Anderson production.
in particular is two minutes of pure John Williams-esque majesty.