I love this one so much. Davies excels at creepy, menacing, difficult-to-comprehend threats, and this is a great example of the genre.

The scene in which the Doctor and Donna split up, only to "reunite" with their fresh doppelgangers is very well-executed, demonstrating that something is amiss without making it too obvious. The scene in which the Doc absolutely cannot stop thinking is great, too.

In terms of ongoing storyline, we have our first Susan Twist appearance as Mrs. Merridew. We also have the origin of "mavity," which has yet to be resolved in the series.

And, of course, we have the Doctor invoking supersition while casting salt at the end of the universe, which is the handwave for the show's new directions into the supernatural. I personally don't think it was necessary, but it is a neat bit of texture.

In fact...do we know if the salt had any bearing at all in the Sutekh story? It kind of seems like the finale could have played out just fine without this element.