The first season of "The Strain," FX's apocalyptic vampire series based on the novels by filmmaker Guillermo Del Toro and novelist Chuck Hogan, was a hit by most measures. Critics generally liked the show, it managed to hold on to strong ratings in its 10 p.m. Eastern slot on Sundays, even when the NFL season kicked off, and it was renewed for a second season.

But while Del Toro was largely satisfied with "The Strain's" maiden run, he is the first to admit that some aspects of the show didn't work quite...