Thought I'd mention another series that has influenced fanfics:
The Dirty Pair.  In 1991 and '92, the Pair accounted for more 
fanfics than any other series (if you include Undocumented
Features).  Admittedly, less and less is being written for them, 
but I expect ADV's release of the OAVs next year may fix that.
Ryan's on this list, he can talk about the influence the Pair
have had on fanfics, especially online.

Other than that, Andrew's essay was right on the money.

--Sean Gaffney
--who, coincidentally, is writing a DP story...imagine!