Genre-defining. For it's numerous bad points, The Discontinuity Guide was the first light-hearted series guide written by fans since the party-line days of Peter Haining, Adrian Rigelsford and the factless Jean-Marc Lofficier. It was written by guys who obviously cared about the series, had moderate knowledge of TV production - by educated fans, for educated fans. Since it's initial appearance in 1995, it has spawned several internet-based imitations, including one which actually bears the same title; as well as starting the ball rolling towards the definitive Who guides: Lawrence Miles & Tat Wood's About Time series.
In a word: Platform.